The official opening night of “Equus,” following three preview performances at East Hampton’s John Drew Theater, was celebrated immediately after the standing ovation Friday night with a Champagne and hors d’oeuvres reception in the walled garden behind Guild Hall, just steps from the stage.

Alec Baldwin (seen here with co-star Sam Underwood) received another ovation when he emerged for the post-curtain party — everyone was already standing as there were few seats in the garden. Baldwin, a star of NBC’s “30 Rock,” plays Dr. Martin Dysart, the psychiatrist who treats Alan Strang, a disturbed 17-year-old stableboy who has blinded horses in a fit of psychosexual conflict.

The disturbed teen is played by Underwood, who drew a crowd of well-wishers when he attended the party — fully clothed (he’s naked for most of the play's final scenes). Also attending were director Tony Walton, a multiple Tony winner, and the author Peter Shaffer, who recently rewrote portions of his 1973 play. This is the first time the revised version has been performed.

Among the celebrity guests were Baldwin’s castmate from “30 Rock,” Kevin Brown (seen here on right at a party Friday night at the Maidstone in East Hampton), who plays Tracy Morgan’s sidekick Dot Com in the sitcom. “This is my first time in East Hampton,” he said, adding that he made the trip to see Baldwin in this dramatic role.

When asked if everyone from the show will be required to see “Equus,” Brown laughed. When Baldwin spotted Brown, he excused his way through the crowd to give him a bearhug.

Shaking hands along the way, Baldwin said to one well-wisher, “This was just our fourth performance. Tomorrow [Saturday] night we’ll really have it down.”

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Also seen at opening night was film director Sidney Lumet (“Dog Day Afternoon,” “Before the Devil Knows You’re Dead”), singer-comedienne Angela LeGreca and Emma Walton, daughter of Julie Andrews and Tony Walton. Along with husband Stephen Hamilton, who plays Alan’s father in “Equus,” she was a co-founder of Bay Street Theatre in Sag Harbor.