A single exposure to the elevated plus-maze test of anxiety reduces or abolishes the anxiolytic efficacy of benzodiazepines. The present study was designed to examine whether this phenomenon of "one-trial tolerance" resulted from a motivational deficit on trial 2. We hypothesized that whereas there is a motivational conflict on trial 1 in relation to the open arms (exploration drive X natural fear of open spaces), there is no "reason" for an animal to explore it on trial 2. A motivational conflict was introduced on trial 2 by rendering the enclosed arms of the apparatus aversive on trial 1. Thus, every time rats entered the enclosed arms, an aversive situation (light and hot air blow) was produced until they left the arm. On trial 2, rats did not receive this aversive stimulation. Chlordiazepoxide significantly enhanced the percent open arm time as well as the percent open arm entries on trial 2 in rats that had been submitted to the aversive stimulation in the enclosed arms on trial 1, but was not effective in rats which had been exposed to the apparatus in the absence of the aversive stimulation on trial 1. In addition, there was no difference in the percent open arm time and entries on trial 2 between saline-treated rats submitted to the aversive or non-aversive condition on trial 1. The aversive condition on trial 1 did not modify the number of total arm entries on trial 2, either. The results suggest that the anxiolytic effect of chlordiazepoxide in the elevated plus-maze depends on the presence of a motivational conflict situation.