A survey of tolerance of a drug, determined in private practice under "naturalistic" conditions by 591 physicians, and involving 22277 patients is presented. The procedure used in private practice to gather systematic information about reactions to the drug involved a system of data sheets with detachable cards for optical reading and computer analysis. The survey was conducted under the responsiblity at regional level of a team of scientific coordinators-hospital pharmacologists and poison control centres. Possible side effects were noticed in 13,82% of patients, a figure similar to known nocebo reactions. Tolerance was significantly related to sex, age, weight, geographical area, duration of treatment, association with other durgs and therapeutic result. When related to individual physicians, the overall number of side effects and the frequency of three of them in particular did not follow a binomial distribution; the rate of adverse reactions was significantly related to the number of years of practice of the physicians.