Protein-deprived rats were given, on one day, a balanced mixture of amino acids followed by access to protein-free food having a distinctive odour. On another day, an imbalanced (histidine-free) amino acid mixture was given just before food having another odour. The rats afterwards preferred the balance-paired odour to the imbalance-paired odour. The preference was acquired whether the duration of odour presentation, or the amount of odourised food taken, was kept constant on the two conditioning days. Retention of the preference seemed unattenuated after 4 weeks. An attraction to the balance-paired odour (relative to odours paired with a water load) contributed to the acquired preference. There was also a relative aversion to unfamiliar odours when they had been paired with imbalance. Such acquired chemosensory control of preferences, together with an anorexigenic effect of imbalanced amino acid mixtures, can account for characteristics of feeding behaviour under conditions in which the diet is deficient in an essential amino acid.

An histidine-devoid but otherwise balanced amino acid mixture depressed food intake from 2 hr after its gastric intubation. It induced conditioned aversion to an odour incorporated in a protein-free diet presented for 6 h following intubation. In other rats, a balanced amino acid mixture established conditioned preference for odour presented in the same diet for 6 h following intubation. The degree of preference was considerably less than the degree of aversion.

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Conditioned odor rejection is induced by pairing with an anorexigenic dose of cycloheximide (.25-1 mg/kg in rats). Thus, an antibiotic not only immediately depresses behavior but also changes later reactions specifically to sensory cues present during its metabolic action. This result relates to regulation of dietary intake and also to experiments on memory biochemistry.

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