1. Water-deprived rats were trained to press either the left or the right bar in a test chamber according to whether they were injected with a central nervous system stimulant or 0.9% w/v NaCl solution (saline). Correct responses were reinforced with water. 2. Different groups of rats learned to discriminate amphetamine or cocaine from saline. Dose-response curves and ED50 values were then determined in brief test sessions when no responses were reinforced. 3. In a crossover study, cocaine was tested in the rats trained to discriminate amphetamine from saline, and vice versa. The two drugs were largely interchangeable, but the ED50 values were increased, indicating a possible, subtle difference in their discriminative stimulus properties. 4. The results indicate the importance of complete crossover designs in combination with dose response determinations when attempting to classify drugs according to their discriminable properties.