Objective: To examine levels of serum tryptophan and its degradation product kynurenine in uncomplicated pregnancy, according to the week of pregnancy and the concentrations of neopterin. Methods: Plasma was analyzed from 45 healthy pregnant women (15 in each trimester), 15 healthy puerperas, and 20 nonpregnant controls. Tryptophan and kynurenine were measured by reverse-phase, high-performance liquid chromatography, and neopterin by radioimmunoassay. Results: In healthy pregnant women, tryptophan values decreased (median first trimester: 72 μmol/L; second trimester: 51 μmol/L; third trimester: 46 μmol/L; P < .001) in a manner correlated with the duration of pregnancy (Spearman rank correlation coefficient rs = −0.771, P < .001) and normalized in the puerperium (median 60 μmol/L). No change in kynurenine, a tryptophan degradation product, was observed, but the ratio of kynurenine to tryptophan increased during pregnancy and correlated positively with gestational age (rs = 0.714, P < .001). In addition, an inverse correlation existed between neopterin and tryptophan concentrations (rs = −0.566, P < .001), as well as a positive one between neopterin and the kynurenine to tryptophan ratio (rs = 0.660, P < .001). Conclusion: Tryptophan levels decrease during normal pregnancy and the decrease may be related to immune activation phenomena.