According to the generalized matching law the ratio of responses between two alternatives matches the ratio of reinforcers produced by these responses. In these experiments using concurrent variable-interval variable-interval schedules in pigeons, responding occurred more frequently on the key associated with the lower reinforcement density (undermatching) than would be predicted by perfect matching. Under control conditions, there was no bias toward responding on either key. Pentobarbital, methamphetamine, morphine and phencylidine all increased bias toward responding on the left key with the exception of one 10 mg/kg dose of pentobarbital that increased bias toward responding on the right key. Higher doses of methamphetamine and morphine, and most doses of phencyclidine increased matching, but high doses of pentobarbital further decreased matching. Morphine increased bias toward responding on the left key at much lower doses than those that affected matching, while phencyclidine increased matching at lower doses than those that increased bias. Pentobarbital produced small increases in response rates that were sometimes accompanied by small increases in key switching. All other drugs only decreased response rate and decreased the number of key switches. These data suggest that drugs disrupt responding under concurrent schedules both by increasing bias and by changing baseline matching functions.