Human behavioral effects resulting from the ingestion of an average dose of diazepam and from 3 h of inhaling either 100 ppm or 200 ppm of methyl chloride (MeCl) were studied in the laboratory. Each of 56 volunteers was randomly assigned to one of six groups comprising the combinations of diazepam and placebo and one of the two levels of MeCl plus control. Each individual was tested in an environmental room on three tasks involving components of eye-hand coordination, mental alertness, and time discrimination. Both pretreatment and treatment data were obtained. Diazepam produced a significant 10% impairment in task performance, whereas the effect of 200 ppm of MeCl was marginally significant (average performance impairment of 4.5%). When the two agents were combined, total impairment was equal to the sum of the individually induced losses. Large interindividual differences in breath and blood levels were found for MeCl.