Heart rate analyses have become an important diagnostic tool for the assessment of autonomic nervous system function. Measurements of heart rate variability (HRV) have been frequently used to establish criteria for a cardiovascular autonomic neuropathy (CAN). Methods of HRV have also been applied to unmedicated patients with psychiatric disorders and to investigate the impact of psychotropic drugs on autonomic cardiac functions. In one of our studies 20% of patients with alcohol dependence (n = 60) fulfilled the criteria of CAN as a result of vagal neuropathy. In other psychiatric disorders clearcut abnormalities of autonomic regulation have not yet been found. However, there is an increasing number of pointers suggesting that patients with panic disorders show a predominance of sympathetic cardiac control, while patients with melancholic depression might show a lack of parasympathetic control. Under the condition of treatment with either 150 mg of a tricyclic antidepressant (TCA) or 300 mg of clozapine per day, HRV-parameters were found to be significantly decreased and the majority of the patients fulfilled the criteria of CAN, whereas the selective inhibitors of serotonin (SSRI) given in usual dosages had no effect on HRV-parameters. In another study with depressed patients treated with TCA, HRV-parameters were found to be helpful in understanding mood-altering chronobiological mechanisms. The results presented make clear that research on autonomic functions is not only useful in neurological diseases, but also in psychiatric disorders especially under the conditions of psychopharmacological treatment.