Ghosh, Tushar K., and S.N. Pradhan: Effects of toluene inhalation on fixed-ratio liquidreinforced behavior in rats. Drug Dev. Res. 11:123-130, 1987. The effect of toluene inhalation was studied on an operant behavior maintained by a fixedratio (FR-24) schedule of liquid reinforcement in rats. A dynamic (flow-through) inhalational behavioral chamber was used to expose rats to toluene vapor. Rats were exposed successively to three graded concentrations (105, 214, 382 ppm) of toluene vapor each for 2 hr in range-finding experiments during 6 114-hr sessions. The reinforcement rate was not altered at hours 1 and 2 but was decreased at hours 3 and 5 of exposure. However, at hours 4 and 6 the reinforcement rate showed recovery from depression. In 2-hr exposure experiments with four different concentrations of toluene on separate days, the reinforcement rate was decreased at hour 1 during exposure to 142,211, and 496 ppm, but exposure to 121 ppm had no effect. There was also no effect when rats were exposed to 115 ppm of toluene during a 5-hr session. The experiment with 2-hr daily exposures to 212 ppm of toluene for 5 consecutive days indicated the development of tolerance to the decreased reinforcement rate during 4th and 5th days of exposure. This study thus shows a minimum effective concentration of 142 ppm of toluene, which would produce no effect on FR liquidreinforced behavior, and indicates that tolerance develops to the behavioral effect of toluene.