1. Marked heterogeneity among species exists in the esophageal response to pharmacological agents. The present study compared the response to serotonin in esophagus from the rat, guinea pig, rabbit and dog. 2. The esophagus from all four species contracted to carbamylcholine and to PGF2 alpha; responses to serotonin were the most variable among species. 3. Serotonin contracted the guinea pig and rabbit esophagus; an effect blocked by LY53857 (10(-7 M) and ketanserin (10(-7) M), consistent with 5-HT2 receptor activation mediating this contraction. 4. Serotonin neither contracted nor relaxed the canine esophagus and relaxed the rat esophagus via 5-HT4 receptor activation as determined by antagonism with ICS 205-930 (-log KB = 6.4), metoclopramide (-log KB = 6.7) and its ester congener SDZ 205-557 (-log KB = 7.9). Two methylene homologs of SDZ 205-557 also had high 5-HT4 receptor affinity (-log KB = 7.7). 5. Thus, in guinea pig and rabbit esophagus, serotonin induced a contraction mediated by 5-HT2 receptors; and serotonin neither contracted nor relaxed the canine esophagus. In rat esophagus, serotonin induced a relaxation mediated by activation of 5-HT4 receptors.