Twenty patients suffering from schizophrenia and 36 patients suffering from endogenous depression underwent a standardized heart rate analysis before drug therapy. The patient\'s parameters of heart rate variability (HRV), which are controlled by the parasympathetic nervous system and which are independent of heart rate, did not significantly differ from the HRV parameters of normal control subjects. Ten of the patients with schizophrenia were treated with 200-400 mg of clozapine/day as monotherapy, while the other ten patients received a combination of different psychotropic drugs. The depressed patients were either treated with 150 mg of amitriptyline/d (n = 24) or 20 mg of paroxetine/d (n = 12) as monotherapy, respectively. After treatment with an average of 300 mg of clozapine/d for 4 weeks or with 150 mg of amitriptyline/day for 2 weeks, all of the patients HRV parameters had significantly decreased (P < 0.001). At this time, about 90% of these patients fulfilled the criteria of cardiovascular autonomic neuropathy. However, treatment with 20 mg of paroxetine/day for 2 weeks had no impact on any of the heart rate parameters. Under amitriptyline treatment, HRV parameters were found to correlate significantly with the plasma levels of amitriptyline/nortriptyline in a group of 104 depressed patients. Thus, determination of decreased HRV parameters is suggested to be a useful tool for the detection of overdosage with amitriptyline. It has not yet been elucidated whether or not the observed HRV decrease, which is probably at least in part due to the anticholinergic side effects of clozapine and amitriptyline, has any impact on patient health.(ABSTRACT TRUNCATED AT 250 WORDS)