Serum relaxin concentrations were quantitated throughout pregnancy. Relaxin levels were higher in the first than in either succeeding trimester of pregnancy. Relaxin concentrations in third-trimester twin pregnancies were not significantly different from those in singleton pregnancies. Relaxin levels in toxemic pregnancies were similar to those of normal pregnancy. In contrast, relaxin concentrations in pregnancies beyond 43 weeks' gestation and in women with premature labor were significantly lower than levels in normal women in the third trimester of pregnancy.