A simple system is described to analyze the possibility that increased exploratory behavior is an index for the anxiolytic effects of benzodiazepines in laboratory rodents. Mice were allowed free run in a two-chambered arena, where two-thirds of the area was illuminated and one-third was darkened. The two chambers were separated by a black partition equipped with photocells across the opening, and the entire cage rested on an Animex activity monitor. Transitions across the partition between the light and dark chambers, and total Animex locomotor activity, were increased by clonazepam and chlordiazepoxide, in dose-dependent ranges consistent with previously reported behavior models. The increased exploratory activity with benzodiazepines does not appear to be a non-specific increase in general motor activity, as locomotion in clonazepam and chlordiazepoxide treated mice placed in a bare, undifferentiated cage was not significantly different from vehicle treated mice.