Abstract
Rates of responding changed systematically across sessions for rats pressing levers and keys and for pigeons pressing treadles and pecking keys. A bitonic function in which response rates increased and then decreased across sessions was the most common finding, although an increase in responding also occurred alone. The change in response rate was usually large. The function relating responding to time in session had the following general characteristics: It appeared early in training, and further experience moved and reduced its peak; it was flatter for longer sessions; and it was flatter, more symmetrical, and peaked later for lower than for higher rates of reinforcement. Factors related to reinforcement exerted more control over the location of the peak rate of responding and the steepness of the decline in response rates than did factors related to responding. These within-session changes in response rates have fundamental theoretical and methodological implications.