The status of spermatogenesis in Snell dwarf, little and congenitally hypothyroid mice was studied. In all of these mice with a hormone deficiency the seminiferous tubules were smaller in size and contained fewer spermatogonia, spermatocytes, spermatids and spermatozoa than did those of normal control mice. There was no substantial difference in the Johnsen score between the hormone-deficient mice and normal control mice, but the former had underdeveloped seminiferous tubules with a corresponding paucity of germ cells, which may be partly responsible for the infertility of these mice. In the present study, growth hormone and thyroxine were administered separately to growth hormone-deficient and thyroxine-deficient mice, respectively. Such replacement therapy brought about an increase in cell counts of the seminiferous tubules and in sperm counts in both groups.