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under ONR Contract N6onr-180, T. 0. IV, Project 143-253, and with support from a grant by the Graduate School of Arts and Sciences. Work is continuing at Harvard University under Contract NSori-07639.

A corollary of the law of effect predicts that the larger the reinforcement, the greater the rate of responding. However, an animal must eat more small portions than large portions to obtain the same daily intake, and one would predict, therefore, that when eating smaller portions an efficient animal would eat less (conserving time and energy) and/or respond faster (conserving time). The latter of these predictions was supported by the present experiments with free-feeding rats for which portion size (pellet size or duration of feeder presentation) and portion price within meals were varied. Response rate was a function of the unit price (responses/g) of food: Rats responded faster when portions were smaller or when prices were higher. Meal size and frequency were relatively unaffected by unit price, but were influenced by the price of meal initiation. The results are discussed in relation to the economic differences between traditional operant and free-feeding paradigms and to both traditional and more recent formulations of the law of effect.

Rats were required to bar press on an ascending sequence of fixed-ratio requirements in order to run in either a voluntary treadmill or a braked running wheel. In both testing situations the duration of the burst of running was controlled by the rat. As the ratio requirement increased the amount of bar pressing increased, reinforcement density decreased, and the duration of the pause following reinforcement increased. These changes in instrumental behavior influenced both the amount of running and the pattern of running. The level of running decreased with the ratio requirement but the average burst duration increased. Increases in running that resulted from deprivation were due to changes in burst duration rather than instrumental performance.

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