Customized Compositions for Chamber : Music: Stephen Mosko’s ‘Psychotropics’ is specifically written for the Southwest Music Society, which will premiere the piece in Costa Mesa.

Composer Stephen Mosko has gone back to a time-honored practice: writing music for people he knows. Haydn did the same for his happy band at Esterhaza Palace in Hungary. Mozart wouldn’t compose a note for an aria until he had met and heard the singer.

Mosko has written “Psychotropics” for five members of the Southwest Chamber Music Society.

“It’s fun to write for people you know,” Mosko said in a recent phone interview from his home in the Santa Clarita Valley. “It’s not so abstract that way.”

“In fact, I chose the people more than the instruments, then figured out--given what they played--how to make the piece work.”

He even tried to get some of the personal and social relationships into the work. Horn player Jeff von der Schmidt and violist Jan Karlin, for instance, are husband and wife. “So they have lots of duets,” he said.

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Flutist Dorothy Stone--Mosko’s wife--has been a friend of oboist Stuart Horn “forever and ever. So they have lots of duets, too.”

The society, which gives the premiere of “Psychotropics” on Saturday as part of the Orange County Performing Arts Center’s chamber series, has played other works by Mosko: “For Morton Feldman” at Chapman University in 1992. Another piece--”Indigenous Music II for Solo Flute”--is scheduled to be played at the Armory Center for the Arts in Pasadena in November. But this is his first commission from the group.

He calls it “a companion piece” to “Psychotropes,” which he wrote a year ago.

“ Psycho means mind, spirit or soul, and trope means to turn ,” said Mosko, long a prominent figure on the local contemporary music scene who has taught for 25 years at Cal Arts in Santa Clarita. “So the title means ‘to change your mind, spirit or soul,’ which I think music always does.”

The title of the new piece came up when he and his wife were thinking about taking a vacation to Hawaii, “which is the tropics, which is the same word as ‘tropes.’ It’s where the Earth turns.”

The idea of turning actually has figured into his composing practices over the last several years, he said. “A certain idea keeps turning back on itself, like a trope turning. As it goes backward, it changes itself, always modulating a little bit. So it’s sort of self-generating.”

The instrumental raw materials for “Psychotropics” are flute, oboe, French horn, viola and piano. “Once you get a combination like that--that very strange tenor range, weird and murky--it defined what I could do,” he said. “So I mostly use the lower range. But it changes all the time. A lot of it is about color and changes of color in the ensemble.”

Those changes all occur within the work’s 12-minute length, which took Mosko about a year to write. “I got interrupted a lot,” he said. “I do so much conducting during the fall and spring. I started a year ago last summer, worked for six weeks, then off and on all year, but it was not solid time.”

He put in a solid 3 1/2 months from May until August, working from an initial outline he likens to “a road map so I know where I am, and then I hardly ever revise. That way, in the morning, I can get up and accomplish what I have to do.

“In a certain way, my music is very much being in the moment, not thinking about where you were or where you’re going. Revising is thinking about where you were.”

* Members of the Southwest Chamber Music Society will play the premiere of Stephen Mosko’s “Psychotropics” on Saturday in Founders Hall at the Orange County Performing Arts Center, 600 Town Center Drive, Costa Mesa. The program also includes Messiaen’s “La Merle Noire” and “Piece” and Bruckner’s String Quintet in F. 8 p.m. $20 , general; $8, students. (714) 556-2787.