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The possible endocrine role of the pineal organ was studied in goldfish. Pinealectomized, sham-pinealectomized and intact goldfish were exposed to various artificially-maintained light regimens during different times of the year and the effects on gonad weight were observed. In addition, the thyroidal and interrenal tissues, the plasma osmotic concentration, and the plasma levels of Ca²⁺, Na⁺ and Cl^– ions were studied in some of the experiments.

The mean gonosomatic index (GSI = gonad weight expressed as percentage of body weight) of goldfish varied throughout the year and was greatest during the month of May. Further, the response of the gonads to increased daily light exposure (e.g. 16 hr. of light and 8 hr. of darkness as opposed to 8 hr. of light and 16 hr. of darkness) was maximal during the 3 months immediately preceding May. These results indicate that whilst gonadal maturation in goldfish is influenced by photoperiod, the efficacy of an increased light—dark ratio in stimulating gonadal maturation is subject to the time of year that it is imposed.

An effect of pinealectomy on goldfish gonad size was restricted to those animals pinealectomized prior to, and held throughout the months of increased gonadal responsiveness to prolonged light (i.e. January, February, March, April and May). During these months, pinealectomized animals underwent greater gonadal development than sham-pinealectomized or intact goldfish when all experimental groups were exposed to 8 hr. light/day.

The thyroidal and interrenal tissues, plasma osmotic concentration, and the plasma levels of Ca²⁺, Na⁺, and Cl^– ions were not affected by pinealectomy in any of the experiments in which these parameters were tested.

The pineal organ of the goldfish was found to be specifically related to gonadal size and this relationship was associated with the responsiveness of the gonads to light.