The effects of prostaglandins (PGs) on melatonin secretion and norepinephrine (NE) and release in rat pineal gland were examined in vitro. To study melatonin secretion, pineal explants were incubated for 6 h in tissue culture 199 medium with 1-1000 nM PGE1, PGE2, or PGF2 alpha. melatonin concentration in pineal glands and media was determined by RIA, PGE2 increased pineal and medium melatonin at all concentrations tested, with a maximum of 1 nM; PGE1 was effective only at concentrations 100-1000 times greater, whereas 100 nM PGF2 alpha gland. Exposure of pineal explants to 10 microM NE brought about a 20-fold increase in melatonin release to the medium. This effect was impaired significantly, but not blocked, by prior exposure to indomethacin, acetylsalicylic acid, or mefenamic acid at supramaximal concentrations to inhibit PG synthesis (100 microM). To examine the effects of PGs on NE release, endogenous NE stores in pineal nerve endings were labeled in vitro by incubating rat pineals with [3H]NE for 30 min. Fifty minutes later, at the time when spontaneous radioactivity efflux had leveled off, transmitter release was elicited by a 1-min exposure to 80 mM K+ (S1), and the stimulus was repeated 35 min later (S2). PGs (10-100 nM) were added to the medium 20 min before S2. Ratios between fractional release of the two consecutive stimulations (S2/S1) varied between 0.84 and 1.16 in control pineals. Only 100 nM PGE2 impaired significantly transmitter release by 40%. These results suggest that PGE2 can play a role in NE-stimulated melatonin synthesis. At greater concentrations PGE2 inhibits NE release from pineal nerve endings.