Correlations between anti-neoplastic activity of medroxy-progesterone acetate (MPA), on the one hand, and serum prolactin (PRL) levels as well as tumour PRL and insulin receptor content, on the other, were investigated in female rats bearing dimethylbenzanthracene (DMBA)-induced mammary tumours. Changes in liver PRL receptor concentrations were also studied. MPA was injected for 15 days. Regression was observed in 16 out of 50 (32%) tumours from rats treated with MPA. Twenty-seven out of 50 (54%) continued to grow regardless of treatment. Stasis was seen in the remaining 7 tumours (14%). Serum PRL levels increased significantly in rats with tumours which were non-responsive to MPA. Concentration of PRL receptors in the liver of all animals was reduced by MPA treatment. A remarkable increase occurred only in those mammary tumours which responded to therapy. The concentrations of PRL receptors in the tumours non-responsive to MPA were similar to those detected in control tumours. Unlike PRL receptors, tumour insulin receptor levels were not modified by MPA treatment, five out of 14 tumours (35.7%), previously growing in spite of MPA administration, regressed when bromocriptine was added to MPA. A significant reduction in serum PRL levels occurred in all rats undergoing the latter treatment. No difference was observed between responsive and non-responsive animals; on the contrary, the PRL receptor content of responsive tumours increased significantly in comparison with that of non-responsive tumours.