Opioid drugs may produce some of their behavioral effects by altering the effectiveness of reinforcing events. The present investigation examined effects of two opioids (methadone and buprenorphine) on pigeons' key-pecking. Different reinforcement rates were arranged using five variable-interval (VI) food-presentation schedules, permitting an application of Heyman's Matching Law analysis and Nevin's Resistance to Change Hypothesis to behavioral actions of opioid drugs. Key-pecking by four pigeons was reinforced by 3-s access to mixed grain under a five component multiple VI schedule. VI values used were 5, 10, 30, 75 and 150s. Each component was in effect for 5min followed by 1min of darkness. Peck rates were high under the VI5-s and VI10-s schedules. As the mean interval value increased, peck rates decreased. Methadone (0.5, 1.5, 2.5, 3.75, and 5.0mg/kg) and buprenorphine (0.25, 0.5, 1.0, 3.0, and 5.0mg/kg), administered 30min prior to experimental sessions, dose dependently decreased peck rates in all subjects. Peck rates under longer VI schedules (75 and 150s) tended to be more greatly disrupted than those maintained under shorter VI schedules (e.g. 10s). Dose-by-dose analysis of key-pecking rate changes under each schedule, and analysis of drug-induced changes in Matching Law parameters suggest that peck rate decreases resulted, in part, from decrements in the reinforcer's ability to maintain behavior.