The authors examined urinary levels of catecholamines and metabolites during a 10-day period of heroin use in 9 subjects. Catecholamine and metabolite excretion increased over baseline values on the first day of heroin use, but markedly different patterns of change emerged later. In contrast to the significant increase in normetanephrine and decrease in metanephrine excretion in all 9 subjects during heroin use, only 4 subjects showed an increase in 3-methoxy-4-hydroxyphenyl glycol (MHPG) excretion. Moreover, it appeared that the increase in MHPG excretion in this subgroup began on the day before heroin administration, which suggests the possibility of an anticipatory or conditioned response.