Hens maintained on 16 h light: 8 h darkness were subjected to an abrupt advance of darkness by between 4 and 13 h. When darkness was advanced so that it occurred less than 6 to 8 h after ovulation, the next expected preovulatory release of luteinising hormone (LH) and ovulation was usually delayed or blocked. This was associated with a reduction in the ability of injected ovine LH to stimulate the secretion of progesterone. When the advance meant that darkness occurred at more than 6 to 8 h after ovulation subsequent ovulations were advanced by as much as 5 h within a 3 d period. The hastening effect of an advance of darkness on follicular maturation was associated with an increased secretion of LH in response to injected progesterone. An advance of darkness by 4 or 7 h was associated with an earlier onset of the circadian increase in the secretion of corticosterone and changes in the tonic secretion of LH, notably a tendency for plasma concentrations of LH to increase during the first 4 to 5 h of advanced darkness, followed by a steep decline.