The effects of oestradiol on prolactin gene expression were studied by quantitative in situ hybridization histochemistry in different prolactin pituitary cell (sub)populations, which had been obtained by separation on a discontinuous Percoll gradient. When cells were incubated in vitro in the presence of oestradiol (10(-8) M) for a period of 4, 24, 48 and 72 h, there was an increase in the amount of prolactin mRNA, from 24 h on, only in high-density prolactin cells and lactotrophs of the total cell suspension. In contrast, the amount of prolactin mRNA in lactotrophs of low density did not change upon treatment with oestradiol. Pharmacological treatment with 50 micrograms oestradiol/day (s.c.) of random cycling female rats in vivo for 14 days increased the total number of prolactin gene-expressing cells and more lactotrophs were recovered at high density after Percoll gradient centrifugation. These results suggest a preferential stimulatory effect of oestradiol on prolactin gene transcription on a subpopulation of lactotrophs. Changes observed in prolactin cell layers after oestradiol treatment in vivo may represent a preferential effect in situ on a particular mammotroph cell subpopulation.