Silent ischaemic ST segment depression and heart rate changes during ambulatory 24 h electrocardiography and bicycle exercise stress testing were studied 8-12 weeks after the first myocardial infarction in 22 patients, aged 36-65 years (X = 50.2). During Holter monitoring it was found that 18 patients had 64 episodes of ST segment depression, including 27 (42.2%) symptomatic and 37 (57.8%) asymptomatic episodes. Mean maximal ST segment depression in painful and painless episodes was 3.5 +/- 1.1 mm and 2.4 +/- 1.2 mm (P less than 0.02), respectively. The mean maximal heart rate in symptomatic and asymptomatic episodes was 112 +/- 14.2 beats min-1 and 115 +/- 16.8 beats min-1 (NS), respectively. No significant difference was found in the duration of symptomatic and asymptomatic episodes. Ambulatory asymptomatic episodes were most frequent between midnight and noon (63.2%). It was observed that when heart rate was below 100 beats min-1 symptomatic episodes were twice as frequent during monitoring while during exercise testing ST segment depression at this heart rate occurred only in two cases. In contrast, there were more asymptomatic episodes at heart rate above 125 beats min-1 and exercise-induced ST segment depression were five times more frequent. One may conclude that silent ischaemia is a frequent phenomenon in the early period after the first myocardial infarction and asymptomatic episodes occur particularly frequently during rapid heart rate (over 125 beats min-1). Episodes of silent ischaemia are found more frequently during Holter monitoring than exercise stress testing.