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Prostaglandins (PGs) can be synthesized by the testes (Ellis, 1972) and are present in the male accessory reproductive tissues and in the semen of various mammalian species (Bygdeman, Fredricsson, Svanborg & Samuelsson, 1970; Tan & Privett, 1972). Their physiological rôle in the male reproductive system is not understood but various effects of PG administration on the testes and male reproductive tract have been reported. These include decrease in plasma testosterone levels (Bartke, Musto, Caldwell & Behrman, 1973; Saksena, Safoury & Bartke, 1973), decrease in testicular blood flow (Free & Jaffe, 1972), and effects on contractions of the testis capsule (Seeley, Hargrove, Johnson & Ellis, 1972), on sperm transport (Hunt & Nicholson, 1972), and on contractile responses of the ductus deferens and seminal vesicles in vitro (Stahl, 1972; Taylor & Einhorn, 1972). In the human, certain cases of male infertility appear to be associated with a low seminal content of PGs

Summary. Plasma prolactin concentrations were determined by radioimmunoassay during oestrous cycles and around the time of oestrus in different types of primitive gilts: Vietnamese, Zlotnicka and wild-boar × domestic pig hybrids. The animals were bled without stress from an indwelling arterial catheter. The following results were obtained: (1) in all gilts the main prolactin peak was observed at Day 15 or 16 of the oestrous cycle; (2) Vietnamese and hybrid gilts showed a second smaller prolactin surge after (Day 2) or before (Day 17) oestrus; (3) base levels of prolactin during the oestrous cycle were 14·8 ± 0·93 ng/ml (Vietnamese gilts), 13·2 ± 1·05 ng/ml (Zlotnicka gilts) and 15·6 ± 2·01 ng/ml (hybrid gilts). The 15–16-day prolactin peaks reached maximum values of 36·4, 43·4 and 56·5 ng/ml respectively.

Recent studies suggest that prostaglandins of the E and F series may cause release of the adenohypophysial hormones, ACTH, growth hormone (GH), prolactin and LH. Evidence for the stimulation of ACTH release from the rat pituitary by prostaglandin has been presented by de Wied, Witter, Versteeg & Mulder (1969); Peng, Six & Munson (1970) and Hedge (1972). Hertelendy, Todd, Ehrhart & Blute (1972) observed that intravenous doses (20 μg/kg) of prostaglandin E₁ (PGE₁) in castrated rams rapidly induced a significant increase in the plasma concentration of GH. Vermouth & Deis (1972) found that two intraperitoneal injections of PGF_2α on Day 18 of pregnancy in rats resulted in a biphasic elevation of plasma prolactin at 12 and 20 hr. Indirect evidence exists which indicates that PGF_2α may stimulate LH release in the rat (Labhsetwar, 1970; Orczyk & Behrman, 1972; Varavudhi & Chobdieng, 1972).

Systemic infusion of PGF