Estrogen causes mammotroph and pituitary tumor growth in vivo; we therefore studied its effect on growth of GH cells, a clonal line of rat pituitary tumor cells. Medium supplemented with gelding serum enabled us to detect effects of estradiol which are masked by the higher estrogen concentrations of commercially available horse serum. GH cells were treated with estradiol for 7 days. Cell number was affected in a biphasic fashion: estradiol at 10(-11) M and 3 X 10(-11) M increased cell number 6- to 13-fold. Concentrations between 3 X 10(-11) M and 10(-9) M caused a dose-dependent decrease from the maximal cell number. The half maximal concentration for this inhibitory effect was about 10(-10) M. Prolactin production was stimulated 3-to 11-fold by estradiol, also with a half-maximal concentration of 10(-10) M. We conclude that estradiol has at least 3 distinguishable effects on GH cells: stimulation of growth at concentrations below 3 X 10(-11) M, and inhibition of growth at higher concentrations, as well as the well known stimulation of prolactin production.