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Concentrations of LH and progesterone were measured in the plasma of ducks which were, from 3 weeks of age, raised on either a constant photoperiod of 16 h light: 8 h darkness or a lighting schedule which simulated natural changes in daylength. In ducks raised on a constant photoperiod of 16 h light: 8 h darkness the plasma concentration of LH increased steeply between 7 and 3·5 weeks before the onset of lay. Concentrations of LH then declined, gradually at first, but then rapidly during the 7 days before the first oviposition in association with a pronounced increase in the plasma concentration of progesterone. During the 18 days before the first egg was laid there was a significant (P < 0·01) negative correlation between the plasma concentrations of LH and progesterone.

The patterns of LH release during sexual development of ducks raised on a schedule which simulated natural changes in daylength were variable but could be categorized according to the daylength at which each duck came into lay. In ducks coming into lay soon after the winter solstice when daylength was short (8·0–8·5 h light/day) there was a pronounced 15-fold prepubertal increase in the plasma concentration of LH although in some ducks high LH levels were not maintained until 3–4 weeks before the first oviposition and were not always followed by a rise in the plasma concentration of progesterone. In contrast, in ducks coming into lay when daylength had increased to 11·0–11·5 h light/day there were only minor fluctuations in the plasma concentration of LH until a small two- to threefold increase in LH was observed during the 2 weeks before the first oviposition.