1. The drug treatment of 1,263 psychiatric patients (= 96% of all in-patients admitted during a period of 2 years) was analyzed by means of case histories. During hospitalization 7.4% of all patients were treated completely drug-free; 12.5% received no psychoactive drugs. 2. The patients were hospitalized for 50 days on the average, and received psychoactive drugs on 43 days. During the total treatment period, they were prescribed an average of 4.3 (median 3.8) different drugs; of these, 2.7 (median 2.3) were various psychoactive drugs. Neuroleptics were the most frequently used drugs; they were prescribed at least once during treatment for 62% of all patients. Antidepressants were prescribed for almost 30% of the patients. 3. For the neuroleptics prescriptions decreased with patient's age; they increased for the antidepressants. Antiparkinsonian agents were prescribed to 51% of neuroleptic-treated patients under 45 years of age, and to 26% of these patients over 45. 4. Neuroleptics were prescribed more frequently in male patients, antidepressants in female patients. 5. Among the individual prescriptions, perazine was the most favoured of the neuroleptics, and amitriptyline of the antidepressants. Clozapine was prescribed for the longest period among all psychotropic agents except lithium salts. Antiparkinsonian agents were used for shorter periods than the neuroleptics administered simultaneously. One-fifth of the patients were given analgesics for brief periods. 6. Medication pattern is related to specific nosology. 95% of schizophrenic patients received neuroleptics for at least an average of 41 days; every second patient in this group received antiparkinsonian agents. 90% of unipolar depressed patients were given antidepressants; during one course of treatment at least 2 different antipressant drugs were prescribed for 32% of these patients. 43% of unipolar depressed patients received neuroleptics for an average of 33 days. Lithium salts were administered for an average of 45 days to 43% of the patients with affective disorders. 7. Treatment surveys of this kind supplement the collection of data on adverse drug reactions by drug monitoring systems. The relationship between the side effects of drugs and the prescription pattern of individual drugs allows one to determine the incidence rates of drug side effects.