Nycthemeral rhythms and sodium chloride appetite in rats

Adrenalectomized rats maintained on a 12:12 light-dark cycle drank large amounts of 0.3 M NaCl solution during the night. They showed virtually no NaCl appetite during the day. As a result of their high day natriuresis, the adrenalectomized rats sustained a negative Na+ balance during the day about three times that of controls. This was offset by a correspondingly higher positive Na+ balance at night. In a second experiment in intact rats, the latency to exhibit NaCl appetite during polyethylene glycol-induced hypovolemia was shorter when the treatment was administered in the evening than in the morning. This again demonstrates a relative facilitation of NaCl intake at night and suggests a major nycthemeral interaction with putative physiological stimuli of NaCl appetite.