The intensity and distribution of histochemically demonstrable dihydrofolate reductase (FH2-R EC 1.5.1.3.) in unfixed cryostat sections was studied in Purkinje neurons of adult vertebrates that have either simple neural circuits and cytoarchitectonics (Ictalurus nebulosus, Rana esculenta) or complex neural circuits and cytoarchitectonics (Coturnix coturnix japonica), compared with the rat as a control. The reaction was generally undetectable in Ictalurus nebulosus and in Rana esculenta; with positive reactions in only a few neurons. On the contrary, FH2-R in the Purkinje cell population of Coturnix coturnix japonica had several pattern (heterogeneity) as in the rat. These results suggest that the existence of FH2-R in Purkinje cell population may be correlated with the complexity of the neural circuits of the vertebrate's cerebellum and that the "heterogeneity" of the reaction may be related to the different functional states of the Purkinje cells within the cerebellum.