The literature supports the concept that circadian changes in body temperature reflect changes in the thermoregulatory set point. We were interested in studying the relationship between the circadian rhythm in body temperature and 24-h variations in plasma concentrations of iron, zinc, circulating leukocyte counts, and plasma interleukin 1 (IL-1) activity. Eight healthy men were studied for two separate 48-h sessions. Rectal temperature, plasma iron and zinc concentrations, plasma IL-1 activity, circulating leukocyte counts, and several other blood variables were monitored. Circadian rhythms in temperature, trace metals, and various leukocyte populations were demonstrated. The 24-h pattern of changes in plasma concentrations of iron and zinc approximate an inverse relationship with rectal temperature. Although we were unable to detect any IL-1 activity in human plasma collected at 4-h intervals, the daily changes in plasma trace metal concentrations and the variations in leukocyte populations may provide indirect evidence for a daily variation in local (e.g., in liver) or central nervous system release of IL-1.