1. Data from intermittent and conventional lighting trials were analys
ed to investigate the effect of daily illumination upon mortality duri
ng the laying period and in 49-d-old broilers. 2. Liveability in layin
g hens was improved by the use of intermittent lighting: the degree of
improvement was proportional to the reduction in daily illumination a
chieved by the intermittent programme. 3. The reduction in mortality w
ith intermittent lighting in laying hens was not the result of intermi
ttent lighting per se. Intermittent regimens which did not reduce dail
y illumination did not reduce mortality. 4. Mortality in both conventi
onally-lit laying hens and 49-d old-broiler chickens increased with ph
otoperiod.