Abstract Six rats were trained to press a lever for a liquid food reward on a multiple fixed ratio-fixed interval (FR-FI) schedule of reinforcement. When lever-pressing rates became relatively stable, the animals were exposed to 150 ppm of either acetone or toluene for duration times of 1 2 , 1, 2 and 4 hr. Exposures were conducted at least three weeks apart. Acetone produced minimal changes on the FR-FI responding during the 1 2 hr exposure. During the 1 hr exposure period, both FR and FI rates increased while during the 2 hr exposure, both FR and FI responses decreased below control levels. During the 4 hr exposure FI responses approximated control levels for 2 rats and were above the control level for the third animal while FR rates were below controls for 2 of the 3 subjects. Rate changes under toluene were generally qualitatively similar to those produced by acetone. An initial enhancement of FR and FI rates occurred during the shorter exposure periods followed by a decrease in rates during the longer exposure periods.