The rhesus macaque represents a pragmatic animal model for elucidating mechanisms underlying normal and pathological human behaviors. Many of the same techniques that are used in clinical studies can be readily applied to the nonhuman primate studies. These including the use of Actiwatch recorders for monitoring of 24-h activity-rest cycles and the use of a remote blood sample collection system for assessment of changes in circadian hormone profiles. In addition, comprehensive rhesus macaque gene microarrays (Affymetrix) are now commercially available, and these can be used for profiling gene expression changes under various physiological and pathological conditions. Our recent application of these methodologies to rhesus macaque studies emphasizes that many physiological and behavioral events, and the expression of associated genes, have a distinct 24-h expression pattern. Consequently, it is important to take these circadian rhythms into account when designing experiments and interpreting the results.