This study compares maternal-fetal transfer and neonatal neurobehavioral effects of opiate analgesics meperidine and alfentanil. Meperidine (pethidine) is widely used in obstetric practice and alfentanil represents a group of newly developed, potent opiate analgesics with pharmacodynamic properties favoring minimal maternal-fetal transfer. The purpose is to provide a thorough and quantitative evaluation of the impact of exposure to opiate analgesic at birth on subsequent well-being of the neonate, and to define the relationship between neonatal effects and the pharmacokinetics of the analgesics in the maternal-fetal-placental unit. This problem is approached in rhesus monkey (Macaca mulatta) which provides a rigorous model for the human obstetric situation through the use of techniques for conscious vaginal delivery in minimally restrained animals, measurement of maternal fetal drug transfer and standardized neurobehavioral assessment of the infant. A very high percentage of the population is exposed to analgesic agents during parturition. Studies using neurobehavioral assessments have raised the question of the safety of these agents for the newborn. However, practical and ethical limits of studies in the human parturant prevent the resolution of many issues. Through this study we hope to provide pertinent, systematic information that will be useful in the selection of analgesics for use in obstetric practice.