The investigation of the fetal factors involved in the initiation and maintenance of labor will be continued using the chronically catheterized fetal sheep with indwelling fetal and maternal vascular catheters. Measurements of the steroids in the delta4 and delta5 pathway as well as prostaglandin F2alpha and its major metabolite 13-14 dihydro-15 keto prostaglandin F2alpha, both in the fetal and maternal circulations will be made before fetal death and in the maternal circulation after fetal death. Changes in uterine blood flow, intrauterine pressure and in myometrial electromyogram activity will also be measured. The sensitivity of the myometrium to exogenous prostaglandin F2alpha and oxytocin infused into either the fetal or the maternal circulation will be tested in late gestation when fetal plasma cortisol concentrations are low and during spontaneous labor at term as well as labor induced prematurely by the infusion of physiological doses of either cortisol or adrenocorticotropin to the fetus. Uterine electromyographic activity and intrauterine pressure measurements will both be used as indices of uterine activity.