The influence of oxytocin and meperidine on responses of the isolated umbilical artery to acetylcholine (ACh) and/or 5-hydroxytryptamine (5-HT) was investigated. 32 preparations from 21 normal full-term deliveries were utilized. The Bunce double-blade hemostat provided an excellent means to collect cord specimens with the least postpartal changes. A method of simultaneously measuring the resistance to flow and longitudinal tension in the perfused artery was found to be a sensitive way to register functional responses. Results showed that oxytocin enhanced the response of the umbilical artery to ACh and 5-HT while meperidine blocked the constriction produced by 5-HT. Both these effects lasted from 30 min to more than 2 h and were reversible.