The effects of 5-hydroxytryptamine (serotonin) and various possible antagonists were tested on the rat uterus, perfused rabbit ear and guinea pig ileum. Of the ergot alkaloids investigated LSD was the most active and specific. On the rat On the rat uterus LSD had about the same activity as atropine has as an antagonist for ACh on the guinea-pig intestine. It is believed that there are two types of specific hydroxtryptamine receptors: a) Receptors in the plain muscle of the rat uterus and rabbit ear which are more easily blocked by LSD than by excess hydroxytryptamine. b) Receptors in the ganglia of the intestine which are more easily blocked by excess hydroxtryptamine than by LSD.