Inhibin-like bioactivity (IBA) was assessed by bioassay in the follicular fluid, ovarian venous plasma, and peripheral plasma of immature gilts in which follicular development had been synchronized by administration of pregnant mare serum gonadotropin (PMSG) and hCG. IBA was detectable in the pooled follicular fluid of control animals, but the concentration as well as the total content increased markedly with the growth of the follicle induced by PMSG. About 36-72 h after PMSG treatment, IBA was found consistently in the venous plasma of both ovaries, although in controls as well as after ovulation (i.e. +48 h after hCG) IBA was not detected. The amount of IBA exiting the ovary by way of venous circulation was only a small fraction (less than 1%) of the total activity calculated to be in each ovary at a given time. IBA was undetectable in the peripheral plasma at any sampling time. On the basis of these results we could speculate that other routes of entry of inhibin into peripheral circulation may exist in the immature pig and/or that the predominant action of IBA might be within the follicle itself.