Three cognitive tasks in which performance depends primarily on the rate of cognitive processing were given to 24 male subjects before and after oral doses of methamphetamine (10 mg), diphenhydramine hydrochloride (100 mg), and placebo. Each subject was tested on Monday, Wednesday, and Friday of one week with drug orders balanced across subjects. Compared with placebo and diphenhydramine, methamphetamine increased the rate at which a visual display was scanned for a target stimulus. Methamphetamine affected neither a time-production task nor a divided attention task that required the subject to perform two cognitive tasks in a limited amount of time. This suggests that methamphetamine can increase cognitive processing speed on tasks involving familiar cognitive operations but that an increase is not likely in tasks involving more complicated decision processes. Compared with placebo and methamphetamine, diphenhydramine caused subjects no experience geophysical time as passing more slowly, but the drug had no significant effects on the visual search or divided-attention tasks. This suggests that time perception is more likely to be altered by diphenhydramine than is performance on tasks requiring short periods of rapid cognitive processing.