We determined the effects of floor surface (wire mesh, WM+, or smooth wood, WM-) and environmental light intensity (20 or 1200 lux) on the exploratory activity of rats in an elevated plus-maze using 15 rats in each group. Rats tested on the wire-mesh floor under low environmental light intensity presented a two-fold increase in the total number of arm entries compared to WM+/high light, WM-/high light or WM-/low light. The relative frequency of open arm entries was greater under WM+/low light (38 +/- 3% vs 20 +/- 5%, 19 +/- 3% and 13 +/- 3% for WM-/low light, WM+/high light and WM-/high light, respectively), as was the percent of time spent in the open arms (27 +/- 4% vs 9 +/- 3%, 8 +/- 2% and 5 +/- 1% for WM-/low light, WM+/high light and WM-/high light, respectively). These behaviors are associated with "aversiveness" as measured with this test. The present results demonstrate that baseline values for drug studies may be decreased or increased by selecting the appropriate type of floor surface and intensity of environmental illumination, thereby permitting more sensitivity, and thus selectivity, for measuring anxiolytic or anxiogenic drug effects, respectively.