The effect of intraventricular administration of prostaglandin (PG)D2 on pulsatile LH release was studied in castrated conscious rats. The administration of 5 micrograms of PGD2 into the lateral ventricle inhibited pulsatile discharge of LH secretion, in contrast to the stimulatory effect of PGE2. Intraventricular administration of 13,14-dihydro-15-keto-PGD2, a metabolite of PGD2, had no significant effect. Intravenous administration of 100 micrograms of PGD2 caused only a slight decrease in LH secretion. Intravenous administration of naloxone, a specific opiate antagonist, blocked the suppressive effect of PGD2 on Lh release. These results suggest that PGD2 plays an inhibitory role in pulsatile LH secretion in castrated male rats and that opiate receptors are involved in the PGD2-induced inhibition of LH secretion.