The in vitro maturation potential of oocytes retrieved during the nonbreeding season with or without prior in vivo low-dose FSH stimulation was studied in adult squirrel monkeys (Saimiri boliviensis boliviensis). Additionally, the adequacy of various protein supplements in media used for oocyte maturation was investigated. Ovaries were removed from animals in the nonbreeding season (n = 9) with or without prior treatment with a low dose (1 mg) of FSH for four days before ovariectomy. Minimal estradiol elevation was observed in serum even with stimulation. For oocyte collection, ovaries were placed in warmed 21 mM HEPES-buffered Ham's F-10. Oocytes from unstimulated ovaries were retrieved and cultured (47 of 62 recovered) in bicarbonate-buffered Ham's F-10 medium containing 0.5% BSA as protein supplement. Negligible maturation was observed at 48 h (3 of 47; 6%), and no fertilization was seen after insemination. Immature oocytes from animals stimulated with a low dose of FSH were cultured (69 of 94 recovered). With prior FSH stimulation, oocytes placed in 0.5% BSA medium matured (13 of 24; 54%) and fertilized (7 of 21; 33%) in marked contrast to oocytes from the nonstimulated monkeys. Additionally, 20% monkey serum and 20% human follicular fluid were studied as alternative protein supplements for the FSH-pretreated oocytes; these produced similar maturation rates (10 of 22, 45%; 10 of 23, 43%, respectively) and fertilization rates (8 of 21, 38%; 6 of 21, 29%, respectively). In some cases, 2 pronuclei were observed at 16 h and 4 cells were observed at 40 h. Of those activated, 63% showed cleavage ranging from 2 to 8 cells by 96 h.(ABSTRACT TRUNCATED AT 250 WORDS)