Inhalation chambers with a monitoring and control apparatus for ethanol vapor exposure of small animals were constructed. A thermal conductivity detector was employed for continuous measurement of inhalation chamber ethanol vapor concentration. The concentration was maintained within a very narrow range (+/- 1 mg/liter) by incorporating into the design a feedback loop which controls the ethanol pump. As expected, the blood ethanol concentrations (BEC) of male Sprague-Dawley rats were positively and linearly correlated to the chamber ethanol concentration. When rats were exposed for 24 hr to a chamber ethanol concentration of 17, 25, or 32 mg/liter, correspondingly low, moderate, or high mean blood ethanol levels were obtained. When a large population of this strain of rats (n = 121) was exposed to a constant ethanol vapor concentration for 14 days (25 mg/liter) considerable interindividual variation in blood levels occurred. There was also individual variation over time in the BEC of animals monitored. The mean +/- SD BEC was 189 +/- 90 mg/100 ml for this population and a gaussian-like distribution was obtained with regard to BEC. Behavior characteristic of alcohol withdrawal was observed in rats with BEC greater than 120 mg/100 ml after 3.5, 7, or 14 days of exposure. This apparatus and inhalation paradigm make possible the precise control of chamber ethanol concentration which markedly enhances control over both intra- and intersubject fluctuation in blood ethanol levels during alcohol exposure and the comprehensive examination of relationships between a wide range of blood ethanol concentrations and their physiological and biochemical effects.