Male and female guinea pigs maintained under controlled lighting and fed ad libitum exhibit a diurnal fluctuation in feed intake. This is characterised by periods of increased eating activity at the beginning and end of the light period and in the middle of the dark period. However, the feed consumption for the light and dark periods is similar. If the normal lighting cycle is retarded by 6 h, the guinea pigs adjust their diurnal rhythm of eating to the new cycle within 8 days. Similar findings in female rabbits have previously been reported from this laboratory.