In a prospective study conducted over 4 months at the Accident and Emergency Department, 21 patients with 24 episodes of spontaneous paroxysmal supraventricular tachycardia (PSVT) were treated with intravenous adenosine at incremental doses of 3 mg, 6 mg, 9 mg and 12 mg until termination of the PSVT. There were 6 males and 15 females. The mean age was 54 +/- 17 years (range from 23 to 83 years). The mean QRS rate was 171 +/- 21 per minute. When data were expressed in a cumulative manner, the response to intravenous adenosine 3 mg, 6 mg, 9 mg and 12 mg in the 24 episodes of PSVT were 5 episodes (21%), 16 episodes (67%), 20 episodes (83%) and 20 episodes (83%) respectively. Adverse effects were present in 10 episodes (42%) of the PSVT. They were all mild and transient, lasting less than 1 minute.