To evaluate the occurrence of arrhythmias and silent ischaemia during a prolonged exhaustive exercise in cold climate conditions, we monitored 37 healthy middle-aged men (age 40-56 years) who were randomly selected from participants of a ski marathon. Completing the 75-90 km race took 7-12 h. The highest and lowest mean hourly heart rates during skiing were 150 +/- 9 (mean +/- SD) and 138 +/- 11 beats.min-1. The maximum heart rate was 161 +/- 9 beats.min-1, and occurred in most skiers during the first hour. Ventricular premature complexes (VPCs) were present in 33 of 37 men (89%) with a median frequency of five beats during skiing (range 0-425). Complex forms occurred in eight men (22%), and atrial ectopics appeared in 33 of 35 participants (94%). The frequency of the arrhythmias did not increase over the skiing period. At control monitoring during a representative period the highest mean hourly heart rate was 74 +/- 12 beats.min-1 and VPCs were seen in 21 men (57%) at a median frequency of one beat during the control period (range 0-338) and complex forms occurred in three men (8%). Three men had asymptomatic ST segment depression of 0.2-0.3 mV lasting 2-10 min during the first hour of skiing. One of them had marginal ST segment depression (0.1 mV) at exercise electrocardiography, but all had normal results at exercise thallium scintigraphy and echocardiography. Thus, arrhythmias were significantly (P < 0.001) increased in middle-aged men during exhaustive prolonged exercise as compared to those observed during a similar period of time of normal daily life.(ABSTRACT TRUNCATED AT 250 WORDS)