Dimethylphenylpiperazinium cyclic peptide synthesis iodide, Deborah methylserotonin hydrochloride, 5,6 dihydroxyserotonin, 5,7 dihydroxyserotonin, D methyltryptamine hydrochloride, N,N dimethyltryptamine hydrochloride and 5 methoxygramine hydrochloride were purchased from Alrich Chemical Co. . SubstancePwas purchased from Bachem Chemicals. Angiotensin II was a generous present from Ciba Geigy. Prostaglandin Dizocilpine MK 801 was a gift from Dr. J. Elizabeth. Pike from Upjohn Chemical Co.. Quipazine maleate was a generous gift from Miles Laboratories. All inorganic salts were analytical grade purchased from Mallinckrodt. Diphenhydramine was purchased from Parke and Davis as a 10 mg/ml ampule. The application of 5 HT to ileum pieces or the longitudinal muscle myenteric plexus planning, caused a dose dependent muscle contraction followed closely by a peace to standard pressure. The scale of the muscular contraction was proportional to the concentration of 5 HT. Enough time for the contraction to achieve basal tension was inversely related to the dose, the highest concentration of 5 HT produced the fastest fade to standard tension. Four min after pretreatment with a dose of 5 HT, a second dose resulted in a diminished contractile response. For an illustration of the trend see fig. 1. A priming dose of 4. 3 X 10M 5 HT displaced the dose result curve to the right in a similar manner without greatly Mitochondrion affecting the maximum response. The blocking aftereffect of 5 HT was fully reversible upon washing. The response curve was shifted even further by higher priming doses of 5 HT of 5 HT to the right, lowering to a minor extent the maximum response achieved. A priming dose of 4. 3 X10 Michael 5 HT entirely antagonized the contractile effects of 5 HT, as evidenced by a flat doseresponse curve. After constant tissue cleanup, recovery of the 5 HT priming dose response in this instance was nearly complete in about 30 min. Analysis of element of this data in a plot unveiled a straight line. The pA2 value for the 5 HT~5 HT interaction was 6. 57 _ 0. 41 and the slope of the point was 1. 59. The pA2 pA10 value was 0. 60. The 5 HT pD2 price established in exactly the same preparation was 6. 52 _ 0. 46. 5 HT in the longitudinal muscle of the ileumexhibitedasimilarauto inhibition effect as that observed in the whole ileum. Application of a dose of 4. 3 X 10 M 5 HT produced a 55 fold increase in the 5 HT Emaxso,, Consistently with the results obtained in the entire ileum, a dose of 4. 3 X10 Mcompletelyabolished5 HT reactions in eight preparations studied. The 5 HT caused car restriction was particular to serotonergic drugs. 4. 3 X 10 M 5 HT.