Regional and sexual patterns in the distribution and density of somatotroph cells exposed to anti-growth hormone serum were analysed by means of immunofluorescence histochemistry in adenohypophyses of normal C57BL mice and abnormal (lit/lit) mutant mice, which exhibit postnatal growth deficits. In adult (3-4 months) lit/lit mice, the regional distribution of somatotrophs both in males and females was normal; however, there was a sparsity of somatotrophs, relative to the normal condition, in the lateral wings of the pars distalis, and sexual differences in the concentration of immunoreactive cells were not as prominent as in the normal mice. In the midline region of the pars distalis a cranioventral zone virtually devoid of somatotrophs occurred in lit/lit as well as in normal mice, especially in the females, though it was not as well defined in lit/lit because of the overall sparsity of somatotrophs. In normal immature mice at 8 and 14 days after birth, the lateral wings did not show the striking sexual differences in density of somatotroph distribution as they did in the normal adults, and at 8 days they were more sparsely populated with somatotrophs than at 14 days. In 14 day lit/lit mice, the lateral wings were less densely populated with somatotrophs than their normal counterparts, but at 8 days these differences were not detectable. In both normal and abnormal 8 day mice, the medial and midline regions of the pars distalis contained less intensely immunoreactive somatotrophs than did the lateral wings.(ABSTRACT TRUNCATED AT 250 WORDS)