Tamoxifen has recently been shown to suppress the growth and secretion of prolactin (PRL)-secreting pituitary tumors in rats. In the present study pretreatment for 36 h with tamoxifen did no modify the suppressive action of bromocriptine on PRL secretion in 2 patients with a prolactinoma and low plasma 17 beta-estradiol levels, while in 2 patients with acromegaly tamoxifen did not influence the lack of effect of bromocriptine on growth hormone (GH) secretion. In one patient with a presumably "mixed" GH-PRL secreting pituitary tumor tamoxifen had a suppressive action on hormone release. This patient had been free of complaints with normalized plasma GH and PRL concentrations for 3 yr with as low as 5 mg bromocriptine per day. Thereafter the pituitary tumor "escaped" from bromocriptine treatment, as evidenced by elevated plasma GH and PRL levels and return of the complaints, despite of an increased daily dose of bromocriptine. Plasma GH and PRL levels increased significantly at the time of ovulation and became lower thereafter parallel with the decrease in the plasma 17 beta-estradiol concentration. Addition of tamoxifen (20 mg/day) to bromocriptine (10 mg/day) therapy had a beneficial effect on the clinical symptoms and normalized the circulating GH levels during a treatment period of 3 months. In patients with pituitary tumors whose hormone secretion "escapes" from bromocriptine treatment, the administration of tamoxifen should be considered. The negative results, obtained with short-term administration of tamoxifen in 4 patients with pituitary tumors and low estrogen levels, do not point to a general usefulness of this drug in this type of patients.