FIG. 2.

Break points maintained by zolpidem, midazolam, cocaine, or alfentanil, in rhesus monkeys trained under a progressive-ratio schedule of intravenous drug delivery. Break point was defined as the maximum response requirement obtained in a session, and the data represent the maximum break points irrespective of dose tested (referred to as “BP _max”). Data are means ± SEM for N = 4 monkeys for each drug, and were obtained from Rowlett et al. (153, 154) and Rowlett and Lelas (152).