Using a newly developed training paradigm, rats were trained to associate a spatial location and a black interior with mild footshock and another adjacent location with white interior with the absence of footshock in three independent experiments. Retention of these associations was measured 24 and 48 hr after training in situation in which the animals could move freely between the black and white locations over a 90 sec test. Other rats were subjected to a control procedure in which shock was received on both the black and the white sides of the apparatus. In each of the experiments, half of the animals in the experimental and the control groups were trained following the administration of the calcium slow channel blocking agent, nimodipine (5 mg/kg), and the other half after saline administration. In experiment 1, the injections were given 15 min before training. In experiment 2, the injections were given chronically, over a 6 day period before training. In experiment 3, the animals were given a single injection 7 days before training. In all cases, retention was examined both 24 and 48 hr after training. The results were that the experimental procedures produce a strong aversion to the black portion of the apparatus. The greatest amount of retention was found in animals that had received the chronic injections, whether they were of saline or of nimodipine. In every aspect of retention in which the saline-treated rats were less than perfect in retention, the nimodipine animals exhibited superior performance.