1. Oviposition times were recorded for brown and white egg-laying hybr
ids under 8, 10, 13 and 18 h photoperiods. 2. Mean oviposition time fo
r both breeds was advanced relative to dusk by approximately 0.5 h for
each 1 h extension of photoperiod. 3. Mean oviposition time for the b
rown egg hybrid was 1.2 to 1.4 h earlier than that of the white egg hy
brid under each lighting regimen. 4. A genetic difference in phase set
ting of the Open Period for Luteinising Hormone (LH) release is the li
kely reason for the difference in mean time of lay of the two breeds.
The difference is possibly one between brown and white hybrids general
ly, rather than between the particular varieties of hen used in this t
rial. 5. The proportion of the day in which eggs are laid is shorter u
nder long photoperiods presumably because light at the end of the phot
operiod inhibits the pre-ovulatory surge of LH.