Intravenous self-administration of the short-acting benzodiazepine midazolam was assessed in rats under conditions of unlimited access to the drug (24 hr/day). Evaluation of the temporal pattern of responding within a session revealed that maximal responding occurred during the dark phase of the 12 hr light/dark cycle. Upon attaining stable rates of responding for midazolam, the rats were tested under extinction conditions (saline). Responding progressively decreased with repeated extinction sessions. Over the course of subsequent reacquisition sessions responding increased, such that asymptotic levels of responding were similar to, or greater than, the rates obtained during the initial acquisition phase. In addition, the total number of sessions required to approach asymptote were fewer than during the initial acquisition phase. Thus, rats given continuous access to midazolam exhibited reliable and stable rates of intravenous self-administration of this short-acting benzodiazepine.