This study compares the speed of onset of action of the extended release (XR) formulation of alprazolam with that of the compressed tablet (CT) formulation in a sample of outpatients with DSM-IV panic disorder. Diary records of hourly antianxiety benefit from a 9-week open label switch study of 30-patients stabilized on alprazolam- CT for 3 weeks and then switched to an equivalent dose of alprazolam-XR, were used to examine the timing and magnitude of clinical benefit on both formulations. The magnitude of benefit at the first hour after the first morning dose was similar for both formulations. The peak benefit, over the hours after the first morning dose, was also similar and 90% of peak benefit that was achieved in the first hour on both formulations. Mean time to peak benefit was similar (1.5 h for alprazolam-CT vs. 1.6 h for alprazolam-XR) and the percent of patients achieving peak benefit in the first hour was also similar. Compared to the CT formulation, alprazolam-XR had a much longer duration of therapeutic action (11.3 +/- 4.2 h vs. 5.1 +/- 1.7 h). The results, which may be related to the biotechnology (and resultant pharmacokinetic profile) of the XR preparation, suggest that alprazolam-XR has value as a "rescue" as well as a prophylactic or maintenance treatment in panic disorder. These results must be viewed in the context of the study limitations including its small size, the lack of independence of groups in a switch study, and the limitations of the diary records used.