Rats' lever pressing was studied on three schedules of reinforcement: fixed interval, response-initiated fixed interval, and fixed ratio. In testing, concentration of the milk reinforcer was varied within each session. On all schedules, duration of the postreinforcement pause was an increasing function of the concentration of the preceding reinforcer. The running rate (response rate calculated by excluding the postreinforcement pauses) increased linearly as a function of the preceding magnitude of reinforcement on fixed interval, showed slight increases for two of the three animals on response-initiated fixed interval, and did not change systematically on fixed ratio. In all cases, the overall response rate either declined or showed no effect of concentration. The major effect of increasing the reinforcement magnitude was in determining the duration of the following postreinforcement pause, and changes in the response rate reflected this main effect.