Understanding the variables that maintain reinforcer effectiveness has important implications for basic research to inform applied behavior analysis. The goal of the current study was to investigate the combined effects of rate of reinforcement and variability in duration and delay of reinforcement on within-session changes in operant responding in two experiments. In each experiment, Wistar rats (Rattus norvegicus) lever pressed for liquid sucrose on three fixed interval (FI) schedules of reinforcement. In Experiment 1, subjects lever pressed for liquid sucrose on either a constant or a varied duration of access to reinforcement. In Experiment 2, subjects were exposed to a constant or variable delay of reinforcement. Results showed two fundamental properties of behavior undergoing habituation. First, within-session decreases in responding were steeper (greater habituation) at higher rates of reinforcement than at lower rates. Second, within-session rates of responding declined more slowly (slower habituation) when access to the reinforcer was presented in a variable versus constant manner. Because habituation occurs for both ingestive and noningestive stimuli, the present study adds to the body of literature on habituation and can inform clinical practice on the variables that sustain operant behavior through varying the delivery of the reinforcer.