AbstractWhen testicular growth is induced in photosensitive white‐crowned sparrows (Zonotrichia leucophrys gambelii) by photoperiodic stimulation, there is an accompanying appearance of acetylcholinesterase‐containing cells in the pars distalis. Such cells do not appear in photorefractory birds subjected to long daily photoperiods; gonadal growth does not occur in these. To study further the function of these AChE‐containing cells, the activities of the thyroid gland, the adrenal cortex, and the gonads were inhibited or eliminated in photorefractory birds. No AChE cells appeared with the induction of increased TSH secretion with treatment of propylthiouracil and thiourea. Similarly adrenalectomy, treatment with metyrapone (2‐methyl‐1, 2‐di‐3‐pyridyl‐1‐propanone) as well as castration of photorefractory birds failed to cause the appearance of these cells. On the other hand, castration of photosensitive males and treatment with long days caused the development of AChE cells which, in comparison with the control, non‐castrated photostimulated males, were hyperplastic and hypertrophied. This supports our earlier suggestion that the AChE cells are gonadot