The eight optical isomers of dimethylheptylpyran as the acetate ester were administered parenterally to normal subjects. The main effect was hypotension which was most prominent when the subjects stood. In the doses used, isomers 2 and 4 (and their mixtures) were the only ones possessing biological activity. Doses of 1 to 2 micro gr/kg produced a pronounced and prolonged orthostatic hypotension. A resting tachycardia occurred with isomer 4. Symptoms were those related to hypotension, plus dry mouth, thirst and hunger, sleepiness, and irritated eyes with conjunctivitis suggesting a mild marihuana syndrome.