鈥� Behavioral and social reactions to intravenously administered heroin were studied during a 33-day experimental addiction cycle. Three groups of four subject volunteers were allowed to self-administer heroin for a ten-day period as part of a longer study of opiate antagonists. Data relevant to sleep patterns, energy expenditure, social interaction, and other observable behaviors were collected during hourly observations. Comparison of behavioral differences before and after drug administration indicated few significant acute reactions. Reactions to long-term heroin self-administration were most pronounced in the areas of sleep behavior and social interaction. Subjects tended to sleep less, especially during the initial period of acquisition, and to withdraw more from social contact. No changes were noted in energy expenditure during waking hours. The results were interpreted in terms of physiological tolerance, central nervous system arousal, and sleep deprivation.