The relationship of catecholamines, urine flow, blood pressure, and other variables were studied in 34 healthy college males placed alone in a laboratory relatively free of external stimuli. Urine was collected pre and post ingestion of water, and the urine flow was found to vary widely in a single individual, both pre and post ingestion of water. The initial urinary flow and excretion rates of norepinephrine (NE) and epinephrine (E) correlated with the degree of water diuresis which occurred within 1 hr. after ingestion of water. The urinary excretion rates for NE and E were also positively correlated with the magnitude of urine flow following water ingestion as well as with systolic and diastolic blood pressure. The heart rate was positively correlated only with E excretion. The baseline NE excretion of smokers was greater than that of nonsmokers. The baseline urine flow of smokers was one-half that of nonsmokers. Serial Nowlis mood-adjective check lists revealed that subjects with a high urine flow checked less anxiety adjectives on arrival. They also checked more adjectives for pleasant feelings at the end of the study and showed an increase in generalized activation items. In view of the recent findings that urinary excretion rates of catecholamines are independent of urine flow and urine pH, it is suggested that the high rate of catecholamine excretion associated with increased urine flow is indicative of elevated plasma levels. These data suggest an interrelationship between autonomic nervous system activity and the response of the individual to an imposed water load. Our most striking finding is that there is a wide variation of response to a simple laboratory atmosphere and that individual differences, present at the very onset of a study, in part determine the subjects' urine flow rates, feeling states, catecholamine excretions, and cardiovascular responses to the laboratory environment and to water ingestion.