Motor impairment (tilt-plane test) test was used to assess the phenomenon of rapid tolerance and crosstolerance to benzodiazepines, barbiturates, and ethanol. The motor impairment responses to benzodiazepines (chlordiazepoxide and diazepam) and to various barbiturates (pentobarbital, phenobarbital, and barbital) were significantly reduced on day 2 in rats that had been treated on day 1 with benzodiazepines and barbiturates, respectively, compared to the control group treated with saline on day 1. Benzodiazepine treatment on day 1 resulted in rapid crosstolerance to the motor impairment effects of ethanol on day 2. Benzodiazepine treatment, however, did not result in rapid crosstolerance to the three barbiturates (pentobarbital, barbital, and phenobarbital) tested. In contrast to the lack of rapid crosstolerance to barbiturates after treatment with benzodiazepines, barbiturate treatment clearly conferred rapid crosstolerance to benzodiazepines and to ethanol. This asymmetry of rapid crosstolerance raises the possibility that benzodiazepines and barbiturates invoke tolerance by mechanisms that are not wholly identical. Therefore, tolerance to the broad range of actions of barbiturates would include crosstolerance to the effects of benzodiazepines, whereas tolerance to benzodiazepines would include only a weak or partial crosstolerance to some of the effects of barbiturates.