Hypophysectomy decreases retinal porphyropsin in the brook trout (Salvelinus fontinalis)

Hypophysectomy of brook trout (Salvelinus fontinalis) results in a decrease in retinal porphyropsin in animals maintained for one month under continuous light. After one month in continuous darkness, retinal porphyropsin decreases to zero percent in all hypophysectomized fish. The effect of hypophysectomy is reversed by treatment with 1 mg/liter of L-thyroxine in the aquarium water for one month. The results indicate that light-induced changes in visual pigment proportions are partly dependent upon factors regulated by the pituitary.