Apomorphine (0.03 mg . kg-1 s.c.) and morphine (0.5 mg . kg-1 s.c.) produced gastric relaxation and vomiting in the conscious dog. Domperidone (0.1-1 mg . kg-1 i.v.), haloperidol (0.1 mg . kg-1 i.v.) and pimozide (0.025 mg . kg-1 i.v.) selectively blocked the gastric relaxation as well as the vomiting caused by apomorphine. Naloxone (0.07 mg . kg-1 i.v.) selectively blocked the gastric relaxation and the vomiting caused by morphine; after naloxone, morphine produced a delayed gastric relaxation in 2 experiments out of 7. These results suggest that for gastric relaxation as well as for vomiting, apomorphine and morphine act on different receptors. Fentanyl elicited marked gastric relaxation, blocked by naloxone, but did not elicit vomiting. After fentanyl, morphine and apomorphine no longer produced gastric relaxation and vomiting. This observation shows that the known blocking effect of fentanyl at the vomiting center does not affect its gastric relaxing effect.