By means of trapping experiments, the times of activity of Apodemus Glethrionomys and Microtus were studied in the field mainly in areas where populations had been marked. The results serve to compare field observations with data already published from controlled laboratory experiments.

Apodemus and Clethrionomys, living in the same habitat, showed a wellmarked nocturnal-diurnal rhythm. In both species their twenty-four hour cycle had its peak around dusk and dawn. Except that there was no two hour period of complete inactivity at Silwood, results in the autumn and winter for Apodemus were similar to those of Elton, Ford, Baker & Gardner (1931). A peak at dusk was followed by a lull and then by a secondary peak persisting until dawn. In the spring and summer, however, when the hours of darkness were short there was no pronounced second peak at dawn.

The feeding cycle of Clethrionomys appeared to be of about two hours duration. As many as 34-9 per cent of the males and 22-9 per cent of the females visited traps at each two hourly trapping period in any one day. The twenty-four hour rhythm of Clethrionomys had, as well as peak activity near dusk and dawn, a further peak in the afternoon. However, when the daylight hours were long, evening activity had often subsided sometime before dusk. In areas where Apodemus and Clethrionomys were both present there were larger catches of the species that had first chance of entry, even when the number of the traps appeared to be excessive. Only a few persistent trap visitors among Apodemus broke their strictly nocturnal habit, such animals being normally only a small percentage of the total population (Brown, 1954). Apodemus took some weeks to range into unoccupied grassland, but once there remained until forced out by high Microtus populations.

Field experiments on Microtus at Silwood confirmed laboratory tests made by Davis (1933). The animal showed a well marked peak of activity at dusk and dawn associated with its twenty-four hour rhythm. It was active both day and night, showing a feeding rhythm of about two hours, but with more activity during the night than during the day.

Except for periods of prolonged frost the three species showed some activity in all weathers. Rainfall might delay appearance above ground or send more animals into the traps for shelter. In heavy rain activity continued to be normal under shelter, but was much reduced in exposed positions.

Factors affecting peak activity including experiments on internal physiological rhythms have been considered. Discussion includes also the possible influence of:-availability of insect food, refection, naturally increased water supplies morning and evening and the avoidance of periods when predatory activity is facilitated, as for example on moonlight nights.