This study was conducted to determine whether prolactin (PRL) suppresses gonadotrophin-induced ovulation and disturbs the co-ordinated gene expression of tissue type plasminogen activator (tPA) and plasminogen activator inhibitor type-1 (PAI-1) in rat ovary. Immature female rats were injected with 10 IU pregnant mare's serum gonadotrophin to stimulate follicle growth, and 48 h received different doses of prolactin followed by 7 IU human chorionic gonadotrophin (HCG). The oviducts were examined for the presence of ova, and the amounts of tPA and PAI-1 mRNA present in the ovary were measured at various times after the hormone treatment. PRL had no significant effect on ovarian weight but caused a dose-dependent decrease in ovulation number. In the control animals receiving HCG alone, 13.3 +/- 1.3 (mean +/- SEM) ova/oviduct were found; while in animals receiving HCG plus 50, 100 or 200 microg PRL, the ovulation number was dose-dependently suppressed by 53.6, 66.9 and 76% respectively at 18 h after treatment. PRL suppression of HCG-induced ovulation was time-dependent. By 24 h after treatment, the number of ova in the oviducts in HCG- and HCG plus PRL-treated groups was not significantly different. PRL also suppressed HCG-induced tPA gene expression in a dose- and time-dependent manner. At all time points examined, tPA mRNA content of whole ovaries and granulosa cells (GC) in PRL-treated groups was lower than in the HCG-treated controls. The activities of PAI-1 in ovarian extracellular fluid (OEF) and PAI-1 mRNA in the theca-interstitial cells (TI) in the PRL-treated groups were higher than in the HCG-treated controls. The highest stimulation by PRL of PAI-1 activity in OEF and of PAI-1 mRNA in TI was observed at 9 h and 6 h after HCG treatment respectively. The localization of tPA and PAI-1 antigens in the ovaries was consistent with changes in the mRNA and activity levels. These data suggest that PRL temporarily delays, but does not completely inhibit, HCG-induced ovulation, which may be caused by a suppression of PA-mediated proteolysis.