Rationale: Illicit drug use can increase driver crash risk due to loss of control over vehicle trajectory. This study asks, does recreational use of +/- 3,4-Methylene-dioxymethamphetamine ( MDMA; ecstasy) and tetrahydrocannabinol (THC; marijuana) impair cognitive processes that help direct our safe movement through the world? Objective: This study assesses the residual effects of combined MDMA/THC use, and of THC use alone, upon perceived trajectory of travel. Methods: Perception of self-motion, or heading, from optical flow patterns was assessed using stimuli comprising random dot ground planes presented at three different densities and eight heading angles ( 1, 2, 4 and 8&DEG; to the left or right). On each trial, subjects reported if direction of travel was to the left or the right. Results: Results showed impairments in both drug groups, with the MDMA/THC group performing the worst. Conclusions: The finding that these psychoactive agents adversely affect heading perception, even in recently abstinent users, raises potential concerns about MDMA use and driving ability.