Triiodothyronine (T3) regulates neuronal development, and T3 deficiency impairs the formation of the central nervous system during childhood. Here, we used the hypothyroidal growth-retarded (grt) mouse to investigate whether the generation of new neurons is affected by T3 deficiency. Cell proliferation in the hippocampus, as measured by 5-bromo-2'-deoxyuridine (BrdU) incorporation, was markedly decreased in the grt mouse while the number of BrdU-positive cells in T3-treated grt mice was equivalent to that of wild type mice. The cellular differentiation rate did not differ among the experimental groups. These results suggest that T3 plays an important role during neurogenesis.