1. Lohmann Brown pullets were reared on 8-h photoperiods and transferred to 12 h at 15 weeks by either advancing dawn or delaying dusk by 4 h. At 25 weeks, half of each group was transferred to 16 h by advancing dawn or delaying dusk and, 10 d later, each photoperiod-group was advanced or delayed 4 h by moving both dawn and dusk. Individual oviposition times were recorded over 48 h at 25 weeks and 8-10 d after each lighting change. 2. At 25 weeks, there were no differences in mean time of oviposition, eggs laid in the modal 8 h, or in the proportion of eggs laid before dawn between pullets that had been given a 4-h increase in daylength at 15 weeks by advancing dawn and those photostimulated by delaying dusk. 3. Extensions of the daylength from 12 to 16 h at 25 weeks, whether by advancing dawn or by delaying dusk, delayed mean oviposition time by 2 h and virtually eliminated egg-laying before dawn. All groups laid > or = 94% of eggs in the modal 8 h. 4. Moving the complete 12- or 16-h photoperiod forwards by 4 h delayed egg-laying by 0.5 h (relative to dawn), whilst moving them backwards advanced it by 0.7 h. Pullets given 16 h of light laid very few eggs before lights-on, but the divergent movements in oviposition time, relative to dawn, resulted in more eggs being laid before lights-on when the 12-h photoperiod was moved backwards and fewer eggs when it was moved forwards. However, a slower adjustment to the new times of dawn and dusk by the pullets whose day had been moved forward, as indicated by fewer eggs being laid in the modal 8 h and poorer rates of lay compared with pullets that had their photoperiod moved backwards, may explain these differences. It is doubtful that the differences would have persisted after all birds had adjusted their ovulatory cycle, and so any reduction in pre-dawn egg-laying is likely to have been transitory. 5. The only permanent way to minimise pre-dawn egg-laying in brown-egg hybrids is to provide a photoperiod of at least 16 h, though 14-15 h may be long enough for white-egg hybrids and 12-13 h sufficient for broiler breeders.