In a discrete-trial procedure, pigeons could choose between 2-s and 6-s access to grain by making a single key peck. In Phase 1, the pigeons obtained both reinforcers by responding on fixed-ratio schedules. In Phase 2, they received both reinforcers after simple delays, arranged by fixed-time schedules, during which no responses were required. In Phase 3, the 2-s reinforcer was available through a fixed-time schedule and the 6-s reinforcer was available through a fixed-ratio schedule. In all conditions, the size of the delay or ratio leading to the 6-s reinforcer was systematically increased or decreased several times each session, permitting estimation of an "indifference point," the schedule size at which a subject chose each alternative equally often. By varying the size of the schedule for the 2-s reinforcer across conditions, several such indifference points were obtained from both fixedtime conditions and fixed-ratio conditions. The resulting "indifference curves" from fixed-time conditions and from fixed-ratio conditions were similar in shape, and they suggested that a hyperbolic equation describes the relation between ratio size and reinforcement value as well as the relation between reinforcer delay and its reinforcement value. The results from Phase 3 showed that subjects chose fixed-time schedules over fixed-ratio schedules that generated the same average times between a choice response and reinforcement.Key words: self-control, amount of reinforcement, delay of reinforcement, fixed ratio, adjusting procedure, key peck, pigeonsWe speak of self-control in situations that involve choice between large, delayed reinforcers and smaller, more immediate reinforcers (Ainslie, 1974). Thus, an animal is said to behave impulsively if it forgoes a larger reinforcer by choosing the reinforcer that is less delayed but smaller. Conversely, it is said to demonstrate self-control if it chooses the reinforcer that is larger but more delayed.We begin with the assumption that an animal's behavior in a self-control situation depends on the relation between a reinforcer's delay and the resulting reinforcement value (i.e., effectiveness in sustaining instrumental responding). A number of different functions relating reinforcement delay and value have been proposed (see Ainslie, 1975). For example, Chung (1965) proposed a negative ex-
