Growth hormone (GH) cells were analyzed by means of ultrastructural morphometry in the pars distalis of pituitary glands from male adult and immature normal (C57BL) and homozygous little (lit/lit) mutant mice. Thin sections were exposed to anti-GH serum and processed immunocytochemically with the colloidal-gold technique. In the pars distalis of adult lit/lit mice, the mean volume density of GH cells/total tissue was 24% of the normal value, granules/GH cells was 58% of normal, and granules/total tissue was only 12% of normal. Deficits in all of these parameters likewise occurred in immature glands, though to a lesser extent than in the adults.The results indicate that the GH deficiency in this mutant reflects quantitative deficits in both the secretory granule content of GH cells, as well as the GH cell content of the gland, with the latter being the more severely affected.