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The purpose of (his study was to determine by RIA the concentrations of relaxin in various compartments and tissues in pregnant women. Ten pairs of maternal venous and cord blood were studied. The mean relaxin concentrations, in immunoreactive equivalents of porcine relaxin, were 0.683 ng/ml in maternal serum and 0.009 ng/ml in cord serum. Relaxin was undetectable in 8 of 9 samples of amniotic fluid. Mean concentrations of relaxin, in pg immunoreactive equivalents of porcine hormone per mg protein, from maternal tissues at term pregnancy were as follows: fat 93 (N = 5), myometrium 47 (N = 4), skin 62 (N = 5), placenta 51 (N = 9), and corpus luteum 13,000 (N = 8). These data suggest that little relaxin crosses the placenta and little is produced in the fetus. Contrary to prior suggestions that relaxin may also be a placenta! product, relaxin appears to be solely produced in the corpus luteum, making it the only peripherally measured hormone that can be used as an index of lutcal activity in pregnancy.