The generalized matching law provides precise descriptions of choice, but has not been used to characterize choice between different doses of drugs or different classes of drugs. The current study examined rhesus monkeys' drug self-administration choices between identical drug doses, different doses, different drugs (cocaine, remifentanil, and methohexital), and between drug and drug-paired stimuli. The bias parameter of the generalized matching law was used to quantify preference for one reinforcer over another. Choice between identical drug doses yielded undermatching. Choices between 0.3 [mu]g/kg/injection remifentanil and either 0.1 [mu]g/kg/injection remifentanil or saline plus drug-paired stimuli revealed bias for the 0.3 [mu]g/kg/injection dose. Choice was relatively insensitive to differences in random interval schedule value when one reinforcer was replaced with drug-paired stimulus presentations. Bias for 0.3 [mu]g/kg/injection remifentanil over 10 [mu]g/kg/injection cocaine was seen in one subject, and indifference was generally observed between 0.1 [mu]g/kg/injection remifentanil and 56 [mu]g/kg/injection cocaine and between 30 [mu]g/kg/injection cocaine and 320 [mu]g/kg/injection methohexital. These findings suggest the bias parameter may be useful in quantitatively measuring level of preference, which would be an advantage over concurrent FR procedures that often result in exclusive choice. (Contains 5 tables and 9 figures.)