Benzodiazepines has been largely used to control anxiety and other clinical pictures since it began to be traded in the 60's. In recent years, many studies have shown that they are used indiscriminately, contributing to increase associated morbidity, and to search for other more accessible tranquilizers. PURPOSE--To estimate the prevalence of benzoadiazepine and over-the-counter tranquilizers use in adults living in Porto Alergre-RS, Brazil. METHODS--480 inhabitants of Porto Alegre, aged 18 or older, selected through a random cluster sampling procedure (sectors of the Brazilian Census), were interviewed in their homes during June, 1991. RESULTS--The prevalence for BDZ use during life, in the past year and in the past month was, respectively, 46.7%, 21.3% and 13.11%. Prevalence was significantly higher among women and widowers or divorced. The majority of those who used BDZ during the past month did so at least 2 or 3 days per week. More than 40% reported frequent over-the-counter substances use to "tranquilize", with a 28.8% prevalence for past month use. CONCLUSION--Chemical substance use to alleviate anxiety and other symptoms is generally spread. Legislation, not always respected, has been insufficient to discipline its use. Understanding the complexity of reasons for the situation to be unchanged is needed. Also necessary is the investment in education on adequate use of BDZ, both for physicians and population, and the search for legal measures that should be more efficient.