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The purpose of the present experiments was to help answer the question of what constitutes a "reinforcing state of affairs" in instrumental conditioning. One of the most systematic positions on this topic (4) holds that ultimate reduction of a survival need is an essential factor, and a closely related position (5) argues for the necessity of ultimate reduction in a primary drive. However, several studies (3, 6, 1)-and at least one theory of reinforcement (10)-suggest the possibility that a sweet taste is reinforcing regardless of whether it is produced by a nutritive or non-nutritive substance. These previous studies demonstrate that sweet-flavored water is preferred to plain water by rats. They do not demonstrate, however, whether a sweet taste, per se, will operate as a reward in instrumental conditioning, uncomplicated either by a measurement of purely reflexive ingestion or by acquired reward from nutritive sweet-tasting substances. Also previous studies apparently have not related the sweet preference to the hunger state of the animal. The present experiments demonstrate that a non-nutiitive sweet-tasting substance functions as a very effective reward in instru mental conditioning. They also demonstrate that the reward value depends on the degree of hunger present.The "reward" used for instrumental conditioning in rats was a solution of saccharine in water. This substance produces a sweet taste in humans which is apparently "satisfying" to its many users (chiefly diabetics and those on reducing diets). The non-nutritive nature of saccharine is indicated by the fact that it apparently goes through the mammalian body unchanged chemically, and animals ingesting it do not diminish their food intake (3). Throughout all the present experiments the solution used was 1.30 grams of pure saccharine powder per liter of water. This value was chosen on the basis of a previous investigation of sweet preference (1) as likely to be effective with most rats.

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