We analyzed plasma samples from 45 randomly selected healthy pregnant women and from 20 healthy nonpregnant female controls of corresponding age. Tryptophan and kynurenine concentrations were measured by reverse-phase high performance liquid chromatography. The kynurenine per tryptophan ratio was calculated to allow a more accurate estimate of tryptophan degradation. In addition, neopterin was measured by radioimmunoassay. There were significantly lower tryptophan concentrations in pregnant women compared to nonpregnant controls. Kynurenine per tryptophan ratios were increased in pregnant women in the 3rd trimester. Significant correlations existed between neopterin increase and tryptophan decrease as well as kynurenine increase and tryptophan decrease. Also significant correlations between week of pregnancy and lower tryptophan and higher kynurenine per tryptophan ratio were found (p < 0.01). From our data, cellular immune activation is likely to be the cause of enhanced tryptophan degradation during pregnancy.