Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit prostaglandin synthesis in humans. Prostaglandins are involved in thermoregulation, melatonin synthesis, and sleep. To determine effects of NSAIDs on body temperature (BT) and melatonin synthesis (MT) in humans, and to elucidate mechanisms by which NSAIDs may alter sleep patterns, a series of experiments using the NSAIDs aspirin and ibuprofen was conducted. Seventy-five subjects were tested under several experimental protocols. BT after NSAID or placebo was assessed in both between- and within-subjects designs at night and during the day. MT levels were assessed after NSAID or placebo at night in a within-subjects design. The normal nocturnal BT decrease was attenuated and MT was suppressed after NSAID relative to after placebo administration. Lower MT levels were associated with a relative flattening of BT. Daytime BT was not affected by NSAIDs. These results are compatible with the hypothesis that some of the behavioral changes associated with NSAIDs, including changes in sleep, are due to changes in BT and MT. We speculate that NSAID effects on sleep and BT are related to prostaglandin synthesis inhibition and/or suppression of MT.