An isotope exchange assay using [3H]progesterone was used to examine progesterone receptor moieties in cytosolic extracts obtained from mammary tissue of gilts over the course of pregnancy and lactation, and during treatment of pregnant gilts with tamoxifen. Scatchard analysis was used to determine the concentrations and dissociation constants of progesterone receptors. The concentration of progesterone receptor was high at the onset of pregnancy (1394 fmol/mg DNA), fell to a nadir at 45 days (36 fmol/mg DNA), increased to a second maximum at 75 days (1232 fmol/mg DNA) and declined thereafter till parturition: the dissociation constant (Kd) of progesterone for its receptor remained stable during pregnancy with a mean Kd of 0.78 nmol/l. Progesterone receptors were not identifiable at day 21 of lactation. Treatment of pregnant gilts with tamoxifen (100 mg or 1.0 g/gilt per day orally identical to 0.70 or 7.0 mg/kg per day) did not affect the development of mammary structures or the ability to lactate at parturition; however, mammary progesterone receptor content in tamoxifen-treated animals tended to be lower than the controls at day 90 of pregnancy (15.7 +/- 1.56 vs 27.0 +/- 3.75 fmol/mg protein respectively). The results show that a temporal relationship exists between oestrogen concentrations in the circulation of pregnant gilts with progesterone receptor content in mammary tissue.