Chronic administration of opiates to laboratory animals induces supersensitivity of the dopamine receptors in the cerebral areas innervated by the mesotelencephalic dopamine pathways. In humans, the in vivo study of the sensitivity of the dopamine neurotransmitter system in Parkinson's patients can be done by means of the apomorphine test, which consists of measuring the number of yawns induced by the subcutaneous administration of low doses of apomorphine (0.005 mg/kg). If chronic opiate use in humans, as in experimental animals, results in supersensitivity of the dopamine systems, the apomorphine test could differentiate between heroin addicts and healthy volunteers, with the former showing greater number of yawns. In order to test this hypothesis we carried out the apomorphine test in two groups of subjects: a group of male heroin addicts attending our Addiction Treatment Centre for detoxification and the other group consisting of healthy volunteer male university students. Results showed that subcutaneous apomorphine administration induced a greater number of yawns (p < 0.05) in the group of heroin addicts as compared with the group of healthy volunteers, suggesting that heroin addicts present an enhanced sensitivity of the dopamine nuerotransmitter system.