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Rates of testicular growth and plasma levels of FSH, LH and testosterone were determined in Japanese quail exposed to various fixed photoperiods (number of hours of light: number of hours of darkness): 12L: 12D, 13L: 11D, 14L: 10D, 16L: 8D and 20L: 4D and to natural daylengths. All five artificial photoperiods stimulated spermatogenesis, with the testes reaching maturity after 30–40 days. Maximum rates of testicular growth occurred with 14L: 10D, 16L: 8D or 20L: 4D but the rate was reduced by 50% in birds exposed to 12L: 12D. This reduction was due to decreased growth in the seminiferous tubule epithelium (and hence in tubule diameter); the duration of spermatogenesis hardly being affected. Near maximum growth rates occurred with 13L: 11D.

The hormone profiles offer an explanation for the differential rates of testicular growth. In the three longest photoperiods, FSH rose from 20 ng/ml to peak levels of 300–400 ng/ml after 10 days. As the testes matured, so the level of FSH decreased to 50–100 ng/ml. This pattern was not seen under 12L: 12D; the level of FSH rose slowly to about 100 ng/ml and showed no peak of secretion. With 13L: 11D a small peak was found, which decreased at maturity. In quail with testes > 1500 mg, the level of FSH was invariably about 100 ng/ml. Patterns of LH secretion were rather similar with all treatments, but testosterone was affected by photoperiod; lower levels were found under 12L: 12D than 20L: 4D. The rate of photoperiodically induced testicular growth was proportional to the levels of FSH, and possibly also testosterone, in the circulation.

Outdoors, testicular growth began when daylengths reached about 12 h. Maturity occurred within the next 40 days. The levels of FSH rose steadily but did not show a peak of secretion. In general, the highest levels of hormone were found in July just before gonadal regression which occurred when the daylengths were still quite long.

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