Seven morphologically and tinctorially distinct types of cell (types 1–) have been distinguished in the pars anterior of the pituitary gland of the musk shrew (Suncus murinus L.). On the basis of their responses to various experimental stimuli, these cell types were correlated with the secretion of various trophic hormones. Type 1 cells exhibited conspicuous changes after thyroidectomy or inactivation of the thyroid gland and hence appeared to be the source of TSH. Types 2 and 3 cells responded to gonadectomy and administration of androgens, which suggests that they were associated with gonadotrophin secretion. The granules of the type 2, but not the type 3 cells could be extracted with 10% trichloroacetic acid, which may indicate that type 2 and 3 cells secrete FSH and LH respectively. After the administration of either reserpine or oestrogen, the type 4 cells underwent hypertrophy and hyperplasia, which suggests that they were the likely source of prolactin. Type 6 cells, which are distinguishable from type 4 cells by their thinly dispersed erythrosinophilic granulation, showed conspicuous changes after unilateral adrenalectomy, administration of metyrapone or exposure to stress and may therefore be responsible for secretion of ACTH. Type 5 cells tinctorially resembled the somatotrophic cells of other mammalian species and did not respond to any of the experimental treatments used in the present study. It is therefore possible that these cells have a somatotrophic function. The possible significance of type 7 cells has been discussed previously.