The core body temperatures of ten healthy young adult and eight healthy aged men were recorded continuously for 14-24 hours using rectal thermometers and analog tape recorders. Aged subjects had significantly higher temperatures at the nadir of the temperature curve compared to the young subjects. Cosinor analysis of the data revealed that, while all subjects' data significantly fit an idealized cosine function of 24 hours periodicity, the peak to trough amplitude of the rhythm was significantly lower in the aged group. No group differences in the 24-hour mean temperature, the mean temperature during the sleep period time, the mesor, or the acrophase were found. These data are in essential agreement with previously reported data but expand upon those findings and demonstrate that the age-related changes in amplitude of the temperature rhythm are not confined to subjects studied under isolated laboratory environments.