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Diurnal changes in hormonal concentrations were determined at various times during sexual development and during the ovulatory cycle of Khaki Campbell ducks raised either on a constant photoperiod of 16 h light: 8 h darkness (16L : 8D) or on a schedule which simulated natural changes in daylength. The ovulatory cycle of the latter group was studied when there were 11 h light/day and daylength was increasing.

In the laying duck concentrations of LH and progesterone in the plasma rose simultaneously between 7 and 4 h before ovulation and oviposition. Preovulatory concentrations of LH and progesterone in ducks subjected to photoperiods of 16L : 8D and 11L : 13D started to increase at the onset of darkness and 2 h after the onset of darkness respectively. In ducks maintained on 16L : 8D there was a rise in the concentration of corticosterone in the plasma at the onset of darkness to attain a maximum just before the onset of light. The temporal relationship of the peak of secretion of corticosterone to the cycle of light and darkness changed during sexual development of ducks raised on 16L : 8D with a 6-h advance in the phase of the rhythm between 11 weeks of age and the onset of egg laying. Highest progesterone concentrations during the day tended to coincide with the maximal secretion of corticosterone. A monophasic diurnal rhythm in the secretion of corticosterone in 11-week-old ducks maintained on natural daylength had disappeared by 15 weeks and was not re-established by 25 weeks of age. A diurnal variation in the plasma concentration of LH was not observed in the ducks maintained on 16L : 8D, although night-time LH concentrations tended to be higher than day-time concentrations in ducks raised on natural daylength. This difference was significant at 11 weeks, and at 15, 19 and 22 weeks changes in the plasma concentrations of LH during the night tended to parallel those of corticosterone.

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