When the annual Esplanade Concerts opened Tuesday evening, 25,000 heat-weary Bostonians turned out to stretch on the cool grass, commune with the innumerable pretty girls, as well as to hear the pleasant, undemanding music played by Arthur Fielder and his Boston Symphony group. Just as the concert began, a full moon rose from behind Boston's buildings, and from the Charles came a light breeze to mitigate the day's blistering heat. As the sky grew darker, and the trees lining the river became black silhouettes, any regular concert-goers present probably were irritated by the rise and fall of chatter and the calls of newsboys stepping over people's legs and bodies. But the Esplanade Concerts are not for these; they are for the man who feels like relaxing at that time of the day and year, who enjoys music unpretentiously and informally, without the restrictions imposed by the discipline, the discomfort of the concert hall.

And the program fitted exactly. Most of it was music that has been listened to with pleasure for fifty to a hundred years, but that it has gathered no cobwebs was shown by the large part of the audience who found trouble restraining from tapping their feet inaudibly on the grass, waving their hands to and fro, or humming partly under their breath. Fielder left classic decorum to the academicians of music, and played with liveliness and humor that did justice to the intention of almost every one of these composers of light concert music. Only the sturdy grandeur of Sibelius' "Finlandia" suffered from the rendition.

The orchestra excelled when they played Richard Strauss' waltzes from "Der Rosenkavalier," and the suite from "Carmen." These two stand-bys put the audience into just the mood into which they wanted to be put, and the two Khatchourian dances, at once delicate and vigorous, led to "Finlandia," which rounded out the first half of the program. After the intermission, Benjamin Britten's variations on Rossini provided a brilliant modern interpretation of this airy, yet worldly, composer.

The concerts, very likely the best entertainment in Boston these days, are being given every night except Mondays at 8:30. Every night the range of the music is immense; tonight's program, for example, includes things by Handel and Johann Strauss. To rent a chair costs a dime, but if you're healthy or sensible, or both, you'll pay nothing and lie on the grass.