Oocyte spontaneous activation (OSA) has been reported to occur during in vitro culture of ovulated rat oocytes. The objective of this study was to compare the rate of oocyte spontaneous activation and the level of maturation promoting factor (MPF) activity in oocytes from different strains. Twelve strains were selected from two commercial sources. Females were superovulated and oocytes collected 17 h after hCG injection. Denuded oocytes were cultured in M16 medium under oil at 37 degrees C and 5% CO(2) in air. The proportion of activated oocytes was determined after 6 h of in vitro culture. Data were compared by analysis of variance (ANOVA), considering each animal as an experimental unit. MPF activity was determined in oocytes from the different strains at 0, 1.5, and 3 h after oocyte collection. The log ratio of the MPF activity at 1.5 and 3 h relative to 0 hours for each animal was analyzed by ANOVA. While significant (p < 0.01) differences were observed between strains in the rate of OSA, there were no differences between strains in the level of MPF during the time points measured (p > 0.3).