Circadian rhythm in the effect of theophylline on the behavioral thermoregulation of the white sucker, Catostomus commersoni.

The temperatures selected by white suckers placed in a thermal gradient were significantly altered by interperitoneal injections of theophylline (25 mg/kg). There was a circadian variation in the effect of theophylline on temperatures selected by individual fish held under constant illumination. Depending on injection time, there was either a significant increase or significant decrease in the temperatures selected. These results are considered in relation to the possible physiological and biochemical effects of theophylline on thermoregulation and circadian rhythms.