The functional status of thyrotrope (TSH) cells was evaluated by means of ultrastructural immunocytochemistry in the adult, early postnatal, and prenatal pars distalis of the pituitary gland in the dwarf (dw/dw) mutant mouse. In the normal (+/+; dw/+) pars distalis, thyrotropes were immunoreactive to anti-TSH serum at all stages studied, although the reaction at 17- and 18 days of gestation was much less intense than that at 26 days postnatally and in the adult. In the dwarf, cells exhibiting ultrastructural features resembling those of thyrotropes were observed, but their secretory granules were unreactive with anti-TSH. At early postnatal stages, thyrotrope-like cells were likewise unreactive in the dwarf, as were the poorly granulated cells present prenatally. Thus, the results indicate that although cells resembling thyrotropes are present in the dwarf, they are functionally abnormal not only in the adult, but also during development.