Concentrations of reverse triiodothyronine (3,3'',5''-T3 or rT3) in amniotic fluid (AF) were measured by radioimmunoassay in 81 patients with various complications of pregnancy and in 39 normal pregnant patients at equivalent gestational age. In normal pregnancy, AFrT3 concentrations decreased with advancing gestational age. At 21-25, 26-30, 31-35 and 35-40 wk of normal pregnancy, AFrT13 concentrations (mean .+-. SE) were 353 .+-. 62, 131 .+-. 49, 94 .+-. 25 and 93 .+-. 5 ng/dl, respectively (ranges: 200-600, 57-350, 66-135 and 50-135). Normal and supranormal values of AFrT3 were found in patients with complicated pregnancy. In patients with Rh isoimmune disease, higher than normal AFrT3 concentrations were associated with seriously affected or gravely ill fetuses, whereas normal AFrT3 concentrations predicted a more favorable outcome. There was a good correlation between AFrT3 and Af pigment levels (r [correlation coefficient] = 0.70, P < 0.001). In complicated pregnancy other than erythroblastosis fetalis, AFrT3 concentrations were not of any prognostic significance, and there was no correlation between AFrT3 and lecithin/sphingomyelin ratio. AFrT3 determination may help in assessment of the fetal condition in erythroblastosis fetalis.