Diurnal body temp. curves of 9 male adults adjusted themselves to a variety of exptl. activity and sleep routines, but not to a rotating activity schedule. In 3 performance tests (color naming, choice reaction time, and Link Trainer operation), the subjects showed a diurnal variation. In general, the higher the body temp., the higher was the performance score. Rank was maintained by each subject in the scores of the 3 tests, but high ranking in one test was often associated with low placing in another. Subjects with higher body temp. levels had higher performance scores in Link Trainer operation and consumed more coffee than those with lower temp. levels. During sleep the body temp. did not fall as low in the summer as it did in the spring, although the temp. and humidity of the air in the sleeping quarters were the same at all times.