Immature females of two inbred strains, KE and CBA, differing in the rate of meiotic maturation, were used in this study. On the 26th day they were treated with 5 IU PMSG (0 h) and 48 h later with 5 IU hCG. Groups of females were killed at 2 h intervals starting with 46 h and ovaries were then dissected for steroid analysis by appropriate RIAs. In both strains hCG injection resulted in a sharp increase in ovarian progesterone content with a simultaneous fall in androgen and oestradiol concentrations. However, in the CBA strain, progesterone concentration started to rise earlier while androgen concentration decreased later than in the KE strain. This subtle difference could be one of the factors contributing to the different maturation rate of KE and CBA strains.