The relation between the rate of a response ("B") and the rate of its reinforcement ("R") is well known to be approximately hyperbolic: B = kR/(R + R[subscript o]), where k represents the maximum response rate, and R[subscript o] indicates the rate of reinforcers that will engender a response rate equal to half its maximum value. A review of data reported in 17 published papers revealed that, under variable-interval schedules of reinforcement, R[subscript o] was usually lower when pigeons were the subjects than when rats were the subjects. The value of k, in contrast, did not differ consistently between pigeons and rats. Some accounts interpret R[subscript o] as the rate of alternative, unscheduled reinforcers in the situation, expressed in units of the scheduled reinforcer. So interpreted, the difference in R[subscript o] implies that less alternative reinforcement (relative to the scheduled reinforcement) typically is available to pigeons in their operant conditioning chambers than it is to rats in theirs. Whether or not that interpretation of R[subscript o] is valid, the pigeon--rat difference in R[subscript o] ensures that for reinforcer rates above about 10 per hour, response rate will be noticeably less sensitive to changes in reinforcer rate (and presumably to changes in other incentive and motivational operations) with pigeons than with rats as subjects, at least with the experimental conditions typically employed. (Contains 1 table, 8 figures and 1 footnote.)