Sexually experienced male CFHB rats, castrated and implanted with testosterone, were chronically treated with domperidone (ca. 5 mg/kg/day). Twice-weekly tests of sexual behaviour showed a significant elevation of both mount latency and ejaculation latency, and suppression of mounting rate after 2–3 weeks of treatment. The number of mounts and intromissions before ejaculation was increased on some tests by domperidone. Experienced male WAG rats, also castrated and testosterone implanted, received three pituitary grafts under the kidney capsule. An increase in ejaculation latency was seen during the first week and then again several weeks after grafting, though implantation of non-endocrine tissue (muscle) into the controls may also have affected behaviour. Both treatments elevated serum prolactin levels, domperidone much more substantially than pituitary grafts. Serum testosterone levels were sustained within the normal intact range by the implants. The weights of the seminal vesicles, ventral prostate and adrenal glands were not significantly altered by either treatment. These experiments show that hyperprolactinaemia induced by two independent methods can retard sexual behaviour in male rats.