The present experiment examines changes in heart rate (HR) and EEG under different doses of marijuana and delta9THC. The design differs from earlier investigation in that (1) the subjects (Ss) were kept constantly alert by being involved in problem-solving tasks; (2) HR and EEG recordings were analyzed by time-locked evoked averaging techniques; and (3) the presentation of each problem task was under computer control and depended on the Ss' return to base-line level of physiological functioning. EEG and ?HR patterns were shown to be significantly related to dose levels and to the different tasks being performed. In addition, the results suggest that marijuana interferes very little with cognitive functions except for impairing short-term memory at all dose levels of marijuana. The procedure described for the measurement of Ss' HR and EEG changes while they are alert as they respond to various perceptual stimuli, and for the analyses of these changes, offers a particularly powerful technique for looking at subtle drug treatment effects not only of marijuana but also of other drugs.