The relationship between plasma kynurenine (KYN) concentration and anxiety and depression in psychiatric patients was examined. KYN has been reported as a probable endogenous anxiogen in animal models of anxiety. Thirty patients with affective states were studied. The Hamilton Anxiety Scale (HAM-A), Hamilton Depression Scale (HAM-D), Dexamethazone Suppression Test (DST) and Diazepam Test (DT) were used to differentiate two groups with prevalence of anxious or depressive symptomatology. It was found that in endogenous anxiety, the plasma KYN concentration is increased and in endogenous depression, it is decreased (resp., 1.94 micrograms/ml and 0.62 microgram/ml, versus normal 0.9 +/- 0.07 microgram/ml). After treatment this concentration became normal in both groups. In this study KYN concentration correlated significantly with the severity of anxiety. The results suggest that the increase of plasma KYN concentration in anxiety may be used as an additional criterion to differentiate endogenous anxiety with depressive mood from endogenous depression in clinical practice. DT and DST have also appeared to be reliable factors in differentiation of anxiety and depression.