Herrnstein's (1970) hyperbolic matching equation describes the relationship between response rate and reinforcement rate. It has two estimated parameters,kandRe. According to one interpretation,kmeasures motor performance andRemeasures the efficacy of the reinforcer maintaining responding relative to background sources of reinforcement. Experiment 1 tested this interpretation of theReparameter by observing the effect of adding and removing an additional source of reinforcement to the context. Using a within‐session procedure, estimates ofRewere obtained from the response—reinforcer relation over a series of seven variable‐interval schedules. A second, concurrently available variable‐interval schedule of reinforcement was added and then removed from the context. Results showed that when the alternative was added to the context, the value ofReincreased by 107 reinforcers per hour; this approximated the 91 reinforcers per hour obtained from this schedule. Experiment 2 investigated the effects of signaling background reinforcement onkandRe. The signal decreasedRe, but did not have a systematic effect onk. In general, the results supported Herrnstein's interpretation that in settings with one experimenter‐controlled reinforcement source,Reindexes the strength of the reinforcer maintaining responding relative to uncontrolled background sources of rein