AbstractMethyltestosterone (MT) (35 μg/l aquarium water) suppresses steroid production in the testes of sham‐operated fish, as studied enzyme histochemically. This treatment also diminishes the volume of the proximal pars distalis, as well as the area of the gonadotropic (GTH) cell nuclei. Such effects were not found in MT‐treated castrated fish. Castration alone led to a partial degranulation of the GTH‐cells as compared with sham‐operated controls. Extensively dilated granulated endoplasmic reticulum was found in the GTH‐cells of some of the castrated animals and also in GTH‐cells of some castrated fish that had been treated with MT. The possibility of a non‐steroidal feedback mechanism from testes to pituitary is discussed. No effects of the treatments were observed on prolactin cells and on the second basophilic cell type (