Putative biological markers of postpartum depression, i.e., plasma tryptophan and other amino acids (leucine, isoleucine, valine, methione, phenylalanine, tyrosine), cortisol, folate, vitamin B12, and urinary biopterins (B) and neopterins (N), were measured in 62 women (mean age 30.8 yrs) in the early postpartum period, 23 pregnant women (mean age 27.8 yrs), and 38 non-gravid controls (mean age 36.2 yrs). The postpartum women were also screened for depression using the Edinburgh Postnatal Depression Scale (EPDS) on day 7 after delivery. Postpartum women had significantly lower tryptophan and vitamin B12 and significantly greater levels of cortisol, folate, N, and B than did controls. Comparisons between women who were classified on the EPDS as cases and non-cases revealed a statistically significant difference only for lower N:B and lower folate ratios in cases than non-cases. Multiple regression analysis showed a significant contribution for low tryptophan to increased EPDS, which also showed significant correlations with low methionine, low tyrosine, low N:B ratio, and high vitamin B12.