This paper deals with the effect of bromocriptine on the morphology and the number of lactotrophs in two inbred strains of mice, one with a high and the other with a low mammary cancer incidence. Bromocriptine decreases the number of lactotrophs in C3H mice while their number in AKR is not affected. These observations suggest that bromocriptine affects the two strains in a different way, probably due to their different prolactin levels in the pituitary. The above treatment resulted also in a shrinking of the RER and a reduction in size of prolactin granules, probably due to a decline in the secretory activity of lactotrophs. Of special interest was the appearance of a 'new' cell type with mixed characteristics of somatotrophs and lactotrophs. These findings, along with an increase in the number of chromophobes, suggest that bromocriptine can induce cell regression. Probably the 'new' cell is an intermediate form between chromophobes and lactotrophs and somatotrophs.