Sexual behaviour of male rats injected with the anti-oestrogen MER-25 during infancy.

Newborn male rats were injected SC with 50, 100 or 200 micrograms MER-25 or 0.05 ml oil daily for the first 10 days of life. As adults, they were tested for male sexual behaviour both before and after castration and replacement with testosterone propionate, and for female sexual behaviour after injections of oestradiol benzoate followed by progesterone. Injections of 100 and 200 micrograms MER-25/day during infancy caused significantly fewer rats to ejaculate than 0.05 ml oil/day in both series of tests for male sexual behaviour. The reduced occurrence of ejaculation could not be related to defective penile development as there was no significant difference in penis weights or the numbers of penile spines between the MER-25 and oil-injected rats. All doses of MER-25 caused significantly more lordosis behaviour after oestrogen and progesterone than did injections of oil. These results provide further evidence that neonatal testicular androgens must be converted to oestrogen in the brain in order to organise male sexual behaviour patterns, including the neural substrate for ejaculation, as well as to suppress female sexual behaviour.