A driver was found asleep behind the steering wheel of his car, and the vehicle was at rest in a traffic lane with the engine running. His manifestations included horizontal and vertical gaze nystagmus, muscle flaccidity, and severe ataxia. He admitted ingesting a white powder, which he identified as an amino acid, about 1 hour prior to discovery by police. A urine specimen collected approximately 1 hour after the traffic stop contained 1975 mg/L of gamma-hydroxybutyrate (GHB). We tentatively conclude that GHB may cause impairment of the psychomotor skills required for safe operation of a motor vehicle.