Thioredoxin (TRX) and glutaredoxin (GRX) are two small proteins catalyzing thiol-disulfide oxidoreductions. A role of both proteins in secretory processes has been suggested and recently it has been demonstrated that thioredoxin functions as a growth factor for lymphocytes in cell cultures. Here we report on the immunolocalization by light microscopy of both proteins in the hypophysis of mammals. We have used affinity purified specific antibodies that give a single band on immunoblots against crude extracts from pig and calf neurohypophysis and adenohypophysis. Thioredoxin was prominently localized in the folliculo-stellatae cells of the adenohypophysis while only a minor proportion of the glandular cells were positive. In the neurohypophysis, thioredoxin immunoreactivity was very intense in the pituicytes and moderate in the clusters of synaptic terminals. Glutaredoxin localization in the adenohypophysis resembled that of thioredoxin whereas in the neurohypophysis there was a clear differential localization: the neurosecretory terminals and Herring bodies were intensely stained for glutaredoxin but not the pituicytes. These results suggest that thioredoxin may be involved in the paracrine modulatory action of folliculo-stellatae cells and that these cells and pituicytes may have similar functions in their respective parts of the hypophysis; the association of glutaredoxin with secretory processes is further documented.