In this research were analyzed the effects of marijuana on human reaction time and on performance for motor responses involving both linear and rotary serial arm movements aimed at a target. A total of six experienced marijuana users served as subjects and three drug conditions (dose levels) were used, i.e., 0, 6.5, and 19.5-26.0 mg delta9-THC. The results showed that (a) (simple and complex) reaction time was not significantly affected by marijuana or by the interaction between drug conditions and the amount of information transmitted during the task, (b) linear movement time was significantly reduced after smoking marijuana, while rotary movement time was not significantly affected, (c) interaction between drug conditions and task complexity was insignificant in the case of both linear and rotary movements, and (d) error rates for the two types of motor movements increased significantly and especially for linear movements as the dose level increased.