Left: Test for conditioned withdrawal in morphine-dependent rats. Values are mean ± SEM percentages of baseline rate of lever pressing/minute. Mean ± SEM pretreatment baseline response rates on the test day were as follows: paired group = 60.0 ± 8.4, unpaired group = 74.7 ± 8.1, saline controls = 95.1 ± 8.7 lever presses/minute. Asterisks indicate results significantly different from the unpaired group and saline controls (p < 0.05). (•, paired group; %unpaired group; n, saline controls). From Baldwin and Koob (1) with permission. Right: Test for conditioned withdrawal in postdependent rats. Values are mean ± SEM percentages of baseline rate of lever pressing/minute. Mean ± SEM pretreatment baseline response rates on the test day were as follows: paired group = 66.0 ± 10.8, unpaired group = 81.9 ± 9.3 lever presses/minute. Asterisks indicate results significantly different from the unpaired group (p < 0.05). (•, paired group; %, unpaired group). From Baldwin and Koob (1) with permission.