A variety of opioid peptides have been described within the vertebrate central nervous system and implicated in the regulation of many rhythmic behavioral and physiological functions. Recent comparative studies point to an early evolutionary development and phylogenetic continuity in the involvement of these opioid peptides in the mediation of basic homeostatic functions. Fish schooling, which can be considered as an early form of vertebrate social behavior, is affected by manipulations of endogenous opioid activity. Furthermore, the extent of opioid involvement in the determination of schooling behaviors undergoes significant day-night variations. The pineal gland, a region of the brain that has been implicated in the determination of circadian rhythmicity in teleost fish, is involved in the modulation of these opioid effects on schooling. It is suggested that interactions between the pineal gland and opioid peptides may be involved in the regulation of rhythmic behaviors and that fish can serve as a useful system to investigate this.