A controlled study was carried out, comparing etomidate and thiopentone used for anaesthesia for cardioversion. Eighty patients presenting with supraventricular tachyarrhythmias were randomly allocated to two groups with stratification for duration of supraventricular tachyarrhythmias and cardiothoracic ratio. After premedication with 10 mg of diazepam, anaesthesia was induced and maintained if necessary with etomidate (0.3 and 0.15 mg.kg-1 respectively) or thiopentone (3 and 1.5 mg.kg-1 respectively). The following observations were made by an independent observer: energy of the electric discharge, systolic and diastolic arterial pressures, heart rate, apnoea with duration, quality of anaesthesia, side-effects if present, and induction, anaesthesia and recovery times. A drawing test and an ambulation test were carried out after recovery. Data were analysed using one-way analysis of variance (continuous variables) and chi-squared test with Yates correction for small numbers (categorical variables) with p less than 0.05 considered significant. The thiopentone (THIO) and etomidate (ETO) groups were similar, except for premedication and number of NYHA class III and IV patients, being more common in ETO (p = 0.04; p = 0.05). Anaesthesia time was 4.6 +/- 2.3 min in THIO and 9.9 +/- 3.5 min in ETO (p less than 10(-9)) with more reinjections in THIO (p = 0.003). Awakening time was shorter in THIO (p less than 10(-6)) and graphic test was carried out better at 15 min in THIO. No difference was found later for graphic test and ambulation. Apnoea was more frequent in THIO (p = 0.002).(ABSTRACT TRUNCATED AT 250 WORDS)