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Amargosa pupfish (Cyprinodon nevadensis amargosae) occupy remote desert habitats that vary widely in environmental conditions from day to night. In this study, diel patterns of behavior were documented for pupfish in their natural habitat, and examined relative to changes in the abundance of mRNAs encoding prepro-arginine vasotocin (pro-VT), prepro-isotocin (pro-IT), three distinct vasotocin receptors (V1a1, V1a2, and V2), and an isotocin receptor (ITR) in the brain. The behavior of wild pupfish varied diurnally, with frequent aggression from 12:00 to 15:00 h and courtship and spawning most common between 15:00 and 19:00 h. Transcript abundance for pro-VT in the brain also changed diurnally with mRNA levels highest at night when the pupfish were least active. Transcripts encoding VT and IT receptors, however, exhibited distinct diel patterns, with V1a2 receptor transcripts showing sex-specific diurnal changes, but V2 receptor and ITR receptor mRNAs varying similarly for males and females. V1a1 and pro-IT transcript abundance were constant over day-night in both sexes. These results document diurnal variation in mRNAs encoding pro-VT and the V1a2, V2, and ITR receptors in the pupfish brain, and provide evidence that diel regulation of V1a2 receptor transcript abundance differs between males and females.