Behavioral toxicity of toluene has been assessed in mice. Because of its small size the mouse can be confined in a 20 l hermetically sealed chamber for several hours. Toluene was introduced through a port and volatilized by a hot-plate. Samples of chamber air for analysis were taken through another port. A smaller mesh cage held the mouse within the larger chamber. Schedule-controlled responding was developed by arranging that a response, breaking a beam of light, was followed by milk under an Fl 60 sec schedule. Responding was much more rapid in the presence of stimuli correlated with the Fl schedule than when the schedule was not operating. Standard sessions consisted of alternating series of 8 consecutive Fl 60 sec and inter-series 30 min time-outs. Toluene disappeared from the atmosphere of the unopened empty chamber at the rate of 0.2%/hr. When the mouse cage was in the chamber the disappearance was 1.5%/hr and when a mouse was also present it was 3.7%/hr. Concentration-effect curves were determined by exposing a mouse to incremental additions of toluene at 30 min intervals. Toluene increased the rate of responding in most mice at levels of about 700 ppm. Higher concentrations progressively reduced responding. The ED50 (the concentration reducing responding by 50%) averaged 1657 ppm in 10 mice. In the appendix, principles for the assessment of hazard from results of this type are presented. It is estimated that there is a 1/1000 chance of the responding of a mouse being reduced by as much as 10% by a concentration of toluene of 69 ppm.