The effect of castration and gonadal steroid replacement on the concentrations of LH-beta and alpha subunit and prolactin mRNA was examined in mice. Mouse LH-beta, alpha and prolactin mRNAs were approximately 0.8, 0.7 and 1.1 kb in size respectively. After ovariectomy, LH-beta mRNA levels increased 2- to 2.5-fold, while alpha mRNA levels increased 2.5-fold 6 and 10 days after ovariectomy. Serum LH rose after 2 days to reach six times control values at 10 days. Pituitary LH content doubled by 8 days after ovariectomy. Prolactin mRNA levels decreased to 50-60% of control at 3, 6, 8 and 10 days after ovariectomy and parallelled the fall in serum prolactin. Pituitary prolactin content fell more slowly, to 50% of intact control values by 10 days. The increase in both LH-beta and alpha subunit mRNA, and decrease in prolactin mRNA, and serum and pituitary hormone changes, after ovariectomy were prevented by oestradiol or oestradiol plus progesterone replacement. Levels of LH-beta mRNA increased more quickly in male than in female mice, the earliest change being seen 24 h after orchidectomy. Maximum values (two- to threefold) were found on day 6 after orchidectomy. Concentrations of alpha mRNA increased by 12 h to between 2 and 2.5 times control from 3 to 10 days after orchidectomy. Serum LH doubled by 12 h and was three to five times greater than control values up to 10 days. Pituitary LH content fell by 48 h before gradually increasing to intact values after 10 days. Prolactin mRNA levels decreased progressively from 2 days after orchidectomy, and this decrease was preceded by a fall in serum and pituitary prolactin which remained low throughout the experiment. Testosterone treatment attenuated the rise in alpha mRNA, prevented the rise in LH-beta mRNA and serum LH and partially restored the decrease in prolactin mRNA seen after orchidectomy. We conclude that in mice, as in rats and ewes, both LH-beta and alpha subunit mRNAs are negatively regulated by gonadal steroids, whereas prolactin mRNA is positively regulated, although there are temporal differences in patterns of mRNA responses between males and females. By comparison with female rats the rise in LH-beta mRNA after ovariectomy was slower in mice. Moreover, the discordant changes in pituitary LH content and LH subunit mRNAs seen in mice after castration were not observed in rats. Furthermore, pituitary prolactin and prolactin mRNA do not fall after orchidectomy of rats.(ABSTRACT TRUNCATED AT 400 WORDS)