In order to quantify autonomic changes related to asymptomatic nocturnal myocardial ischaemia, we analyzed heart rate fluctuations recorded during Holter monitoring in 9 subjects with coronary heart disease (21 episodes) and in 11 age-matched controls. R-R interval spectral analysis was computed in sequences of 256 heart beats, taken during the ischaemic episode, 4, 8 and 60 minutes before, and 4 and 60 minutes after. Mean heart rate, R-R interval variability (assessed by R-R interval standard deviation), low and high (respiration-linked) frequency components of R-R interval spectrum were evaluated. Mean heart rate and R-R interval variability increased only during ischaemia (from 62.9 to 73.3 beats/minute, P less than 0.02, and from 39 to 88 msec, P less than 0.01, respectively). While high-frequency components of heart rate variability remained unchanged, low-frequency peak increased during ischaemia (from 9.4 to 43.3 sec2 X 10(-3)/Hz, P less than 0.01) and also 8 minutes (P less than 0.05) and 4 minutes before (P less than 0.05). Despite a moderate increase of heart rate occurring only during ischaemia, the early rearrangement of heart rate fluctuations suggests the occurrence of changes of autonomic tone before the electrocardiographic onset of ischaemia. Due to its limited amount, this phenomenon appears to be a consequence, most likely unspecific, of factors responsible for the genesis of myocardial ischaemia.