AIM: To assess how price and income affect hypothetical drug-purchasing decisions of polydrug abusers undergoing treatment for heroin addiction. PARTICIPANTS, DESIGN AND SETTING: Forty subjects participated in experiments in which they hypothetically "purchased" drugs as price or income varied. INTERVENTION: Experiment 1 examined effects of heroin price on purchases of heroin, valium, cocaine, marijuana and alcohol. Experiment 2 examined the effects of both heroin and valium prices on purchases of these drugs. In both these experiments, income remained constant. Experiment 3 examined the effects of increasing income on drug choices, with drug prices constant. FINDINGS: As price of heroin rose in Experiment 1, heroin purchases decreased. Reductions in heroin purchases were proportionally less than price increases, demonstrating inelastic demand for heroin. Valium and cocaine purchases increased as heroin price rose, and cross-price elasticity coefficients indicated that these drugs substituted for heroin. In Experiment 2, demand for both heroin and valium was inelastic. Valium substituted for heroin, but heroin purchases were independent of valium prices, suggesting an asymmetrical substitution effect. Marijuana and alcohol purchases were independent of valium price, but both these drugs substituted for heroin. In Experiment 3 demand for heroin and cocaine was income elastic, with purchases rising in greater proportion than income. Marijuana, alcohol and valium purchases did not vary with income, demonstrating that demand for these drugs was income inelastic. Hypothetical choices were reliable both between and within subjects. Moreover, drug choices in this hypothetical situation were correlated with urinalysis results, demonstrating initial validity of this methodology. CONCLUSIONS: This methodology may be useful for understanding the phenomenon of polydrug abuse.