Epithelial cell-rich fractions of rat mammary gland were prepared using percoll gradients after collagenase dispersion. Their insulin-binding characteristics were similar to those of crude acini but superior to unfractionated isolated cells. Optimal binding was obtained after 60 min at 20 degrees C or 16 h at 4 degree C at pH 7.8 Binding at 37 degrees C was lower due probably to an enhanced rate of insulin degradation. 48 h after ovariectomy of 18-day pregnant rats insulin binding to acini doubled due to an increase in the number of insulin receptors. Progesterone but not bromocriptine (which prevented the rise in serum prolactin which occurred after ovariectomy) prevented this increase in insulin binding. These results illustrate that the change in serum progesterone rather than prolactin increases insulin binding to the mammary cell at parturition whilst insulin binding decreases in adipose tissue at the same time (Flint et al., Mol. Cell Endocrinol, 20, 101-111, 1981) enabling coordinated changes in the metabolism of these 2 tissues to take place during the perinatal period.