Tests consisting of 20 trials each for simple reaction to light and to sound stimuli and choice reaction to light and to sound stimuli were made from 5 to 7 times daily on 5 adult human subjects. Reaction times were measured by means of a Hipp chronoseope, and the oral temp. at the time was determined with a clinical thermometer. There was observed a diurnal variation in simple and choice reaction times, with best performance in the afternoon and poorest early in the morning and late at night. This variation appears to be dependent upon the diurnal body temp. curve being maintained. A spontaneous or induced change in body temp. is reflected in a change in the opposite direction of reaction time. The influence of changes in body temp. is more pronounced upon complex than upon simple processes and performances. As a result, there is a greater diurnal variability in reaction velocities of complex processes, such as mental work, and the temp. characteristics for these processes are greater than are those for simpler biological processes.