Two experiments were conducted investigating the effects of a 24-h CS-US interval in taste aversion learning. In the first experiment, male albino rats were allowed to consume a nonpreferred CS (HCL-flavored water) and injected, intraperitoneally, 24 h later with an illness-inducing drug (cyclophosphamide). After two such pairings, the animals exhibited an aversion to the flavored water. In Experiment II, female rats were exposed to the same US 24 h following consumption of a preferred CS (saccharin-flavored water). However, after only one such pairing, experimental Ss avoided the flavored water and preferred normal tap water in a two-bottle choice situation, while saline-injected controls preferred the saccharin-flavored water to the plain tap water. Due to the controversial nature of conditioning with such long delays of reinforcement, a systematic replication of Experiment II was conducted with male rats, and again an aversion was established to the saccharin-flavored water.

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In an experiment to determine the optimal time of day for the establishment of taste-aversion learning, 24 male albino rats were divided into four equal groups. Following 14 days of ad-lib water and food, Group 1 animals were water deprived beginning at 4 a.m. on Day 15. Group 2 was also introduced to water deprivation on Day 15 beginning at 10 a.m.; likewise, Group 3 began at 4 p.m. and Group 4 started at 10 p.m. After 24 h of deprivation, the taste cues (1.5 grains of saccharin in 600 cc of tap water) were made available to each group at 4 a.m., 10 a.m., 4 p .m., or 10 p.m., respectively. After allowing the animals to drink the taste cue for 1 h, the drinking tubes were removed, and, subsequent to a 30-min interval, each animal was injected with cyclophosphamide, an illness-inducing drug. The subjects were then given 4 days to recover from the ensuing illness with water available ad lib. Upon recovery, the animals in each group were again water deprived for 24 h using the same times as on Day 15. The next day, all groups were tested for aversions at 4 a.m., 10 a.m., 4 p.m., or 10 p.m., respectively, by giving each animal in each group access for 1 h to two bottles: one filled with tap water and the other filled with the taste cue. Results indicated that aversions were strongest for the 4 p.m. animals and weakest for the 10 p.m. group. The 4 a.m. and 10 a.m. animals fell between the other two groups in terms of aversion strength.

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