The prehospital treatment of paroxysmal supraventricular tachycardia (PSVT) with verapamil alone or in comparison with other interventions has not been studied. A sequential protocol consisting of Valsalva maneuver, ice packs, pneumatic antishock garment inflation, and verapamil 0.5 mg IV was implemented in an urban emergency medical services system after appropriate paramedic education. On-line physician discretion could alter the protocol in specific clinical situations. Inclusion criteria required a narrow-complex regular tachycardia at a rate of between 150 and 250. Of 43 patients identified, the protocol was applied sequentially in 26 (60%) and with valid physician discretion in 15 patients (35%) (due to hypotension, chest pain, or dyspnea). Conversion to a more stable rhythm occurred in 25 patients (59%). Of these, all but one achieved a sinus mechanism. The Valsalva maneuver and ice pack application were ineffective in achieving rhythm conversion, being successful in only two of 33 (6%), one of 31 (3%), and four of 24 (17%) attempts, respectively (P greater than .05). Verapamil, however, was significantly more effective (18 of 24 attempts, 75%) than any of the other interventions either singly (P less than .01) or in combination (P less than .0001). No clinically important adverse effects occurred with any intervention, but application of the protocol increased mean on-scene time to 30.6 minutes. We conclude that verapamil is an effective agent for the prehospital termination of PSVT but that careful on-line physician supervision and a system for confirming rhythm identification are both essential.