The plasma catecholamine response to a standardized bicycle exercise test was evaluated in 24 insulin-dependent diabetic (IDDM) patients in whom the heart rate reactions to deep breathing (E/I ratio) and to tilt, the immediate acceleration and the transient deceleration (acceleration and brake indices), had been assessed as tests of autonomic neuropathy. Patients with an abnormal acceleration index (n = 8) showed, compared with non-diabetic (n = 18) controls who had participated in previous studies, an impaired increment in noradrenaline during exercise (80% of maximal working capacity) (MWC) (12.38 +/- 1.46 nmol l-1 vs. 18.74 +/- 1.45 nmol l-1; P less than 0.01) and adrenaline (50% of MWC: 0.25 +/- 0.04 nmol l-1 vs. 0.54 +/- 0.08 nmol l-1; P less than 0.05). Similarly, patients with an isolated abnormal brake index (n = 6), i.e. with a normal acceleration index and a normal E/I ratio, showed compared with controls an impaired increment in noradrenaline (9.53 +/- 1.66 nmol l-1 vs. 18.74 +/- 1.45 nmol l-1; P less than 0.01) and adrenaline (1.41 +/- 0.22 nmol l-1 vs. 2.92 +/- 0.51 nmol l-1; P less than 0.05) during 80% of MWC. IDDM patients with abnormal heart rate reactions to tilt, an abnormal acceleration index or an abnormal brake index show impaired catecholamine responses to exercise, which can be demonstrated also in patients without signs of parasympathetic neuropathy.