MDA (2.0 and 2.2 mg/kg) was compared to LSD (10 microgram/kg) and d-amphetamine (3.2 mg/kg) in single dose, antagonism, cross tolerance and appetite suppression studies. In single doses MDA specifically resembled d-amphetamine by producing marked mydriasis, nictitating membrane retraction, stereotypy and darting eye movements and LSD by markedly facilitating the flex or reflex, producing continuous stepping, whining and eye tracking movements. LSD and MDA increased respiration, body temperature and the latency of the skin twitch reflex and produced behavioral arousal. Cyproheptadine antagonized the effects of LSD but was ineffective against MDA. Phenoxybenzamine antagonized the respiratory, pupillary and hyperthermic effects of MDA and the respiratory effect of LSD. Chlorpromazine antagonized many of the effects of LSD and MDA. Spinal dogs were made tolerant to the behavioral and physiologic effects of LSD. Cross tolerance developed to some but not all of the effects of MDA. In intact dogs MDA was 1/10 as potent as d-amphetamine in suppressing appetite. It is concluded that MDA has properties resembling both LSD and amphetamine.