The cardiac rate and rhythm were studied by 24-hour ambulatory electrocardiographic recording in 44 patients before, during, and after being discharged from hospital following an acute myocardial infarction. The first recordings were started 48 hours before discharge, the second on the morning of the day of discharge, and the third 48 hours after discharge (at home). While in hospital and after returning home the heart rate fell during sleep but there was no diurnal variation in the frequency of ventricular extrasystoles. Daytime heart rate and both the frequency and grade (severity) of ventricular arrhythmias were significantly raised 48 hours after discharge. The frequency of ventricular extrasystoles during sleep was also increased in the 48 hours post-discharge recording. Rises in heart rate and frequency and severity of ventricular extrasystoles were observed on the morning of the day of discharge, increasing up to the time of leaving hospital, but during the journey home they all diminished. No relation was found between ventricular arrythmias during early convalescence and (i) ventricular arrhythmias during the acute phase of acute myocardial infarction (including ventricular fibrillation); (ii) peak aspartate aminotransferase; (iii) the level of anxiety; or (iv) the personality type. Six patients taking beta-blocking drugs behaved similarly. Five patients taking anxiolytic drugs has significantly raised frequency of ventricular extrasystoles during each 24-hour electrocardiogram. In spite of the above findings, at the time of leaving hospital after acute myocardial infarction there does not appear to be a serious risk from the development of major cardiac arrhythmias.