Facing mounting pressure on campus and in the national press, University President Lawrence H. Summers issued last night his most extensive apology since suggesting on Friday that “innate differences” between the sexes may account in part for the underrepresentation of women on elite science faculties.

“I deeply regret the impact of my comments and apologize for not having weighed them more carefully,” Summers wrote in an open letter to the Harvard community.

“I was wrong to have spoken in a way that has resulted in an unintended signal of discouragement to talented girls and women,” he added in the six-paragraph statement posted on his website. “As a university president, I consider nothing more important than helping to create an environment, at Harvard and beyond, in which every one of us can pursue our intellectual passions and realize our aspirations to the fullest possible extent.”

The apology came after a three-day media frenzy and a barrage of criticism from academics at Harvard and across the nation.

Immediate reaction to the apology was mixed. Student leaders of campus women’s groups praised the president’s statement, but two members of the Faculty Standing Committee on Women said the letter would not heal the wounds still stinging from Summers’ remarks last week at a National Bureau of Economic Research symposium.

The letter “puts a huge smile on my face,” said Connie Zong ’05, co-vice president of Women in Science at Harvard Radcliffe.

But Summers’ statement failed to assuage anger among Standing Committee members, who yesterday sent a letter to the president blasting him for sending “at best mixed signals to our high-achieving women students.”

“I fear it’s going to take much more than a letter to repair the damage that’s been done,” said Kay K. Shelemay, the Watts professor of music.

Professor of Economics Caroline M. Hoxby ’88 said the letter “doesn’t really change the environment for graduate students who feel their ability has been questioned and for undergraduates who are taking exams.” She said the Standing Committee planned to meet this morning.

Melissa E.B. Franklin, the Mallinckrodt professor of physics, who has been one of the president’s most vehement critics this week, called Summers’ letter “a great step forward.”

“There are probably quite a few members of the Harvard faculty who actually believe that there is an innate difference between girls and boys in science skill,” Franklin said. “Now that this is brought up, maybe all the ensuing discussion will be able to convince those people otherwise.”

Nancy Hopkins '64, the MIT biologist who first objected to Summers' remarks and reported them to the media, said today she was nonplussed by the president's apology.

"One still has to be deeply concerned about what he said on Friday. Unfortunately, there really isn't yet any convincing evidence that he was misunderstood," Hopkins said.

As students hunkered down for the final stretch of exams, word of Summers’ letter moved slowly across campus.

The two co-chairs of the Radcliffe Union of Students, Dara F. Goodman ’07 and Giselle B. Schuetz ’07, both said they hoped Summers would follow up his words with concrete actions to bolster the position of female scientists at Harvard.

“I think he made a lot of positive strides,” said Goodman, “but the next step he can take to prove his commitment to women in the sciences is to start tenuring them.”

Schuetz, a history of science concentrator, said that she and many of her friends e-mailed Summers individually to express their frustration with his remarks.

In his letter last night, Summers wrote that he “had learned a great deal from all that I have heard in the last few days.”

“The many compelling e-mails and calls that I have received have made vivid the very real barriers faced by women in pursuing scientific and other academic careers,” Summers wrote. “They have also powerfully underscored the imperative of providing strong and unequivocal encouragement to girls and young women interested in science.”

At the request of symposium organizers, Summers delivered a luncheon talk to 50 academics in which he presented several speculative hypotheses to explain the scarcity of female scientists at top universities. Summers has said that he did not expect the remarks to be published.

Last night’s apology came on top of a letter Summers sent responding to the Standing Committee Tuesday night in which he wrote that he “had hoped to stimulate research on many interrelated factors that bear on women’s careers in science.”

“I misjudged the impact of my role as a conference participant,” he wrote Tuesday.

Calls to Summers’ spokeswoman were not immediately returned last night.

The full text of Summers’ letter is available online at www.president.harvard.edu.

—Staff writer Daniel J. Hemel can be reached at hemel@fas.harvard.edu.

—Staff writer Zachary M. Seward can be reached at seward@fas.harvard.edu.