Cerebrospinal fluid (CSF) and blood concentrations of indoleamines and catecholamines were analyzed in 14 HIV-1-seropositive individuals before antiviral treatment with zidovudine, after 3-14 months of treatment, and, in 8 of the patients, also after 14-30 months. The median pretreatment concentrations of tryptophan in CSF and blood were low (224 ng/ml and 6.0 micrograms/ml, respectively), but an increase in these values by an average of 40% in CSF and 23% in blood was seen after 3-14 months of zidovudine treatment (p < 0.01) and remained undiminished after 14-30 months of treatment. No significant change was observed in the 5-hydroxytryptamine (5-HT, serotonin) level in blood or in the CSF concentrations of the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA) and the dopamine metabolite homovanillic acid (HVA). The CSF concentrations of the noradrenalin metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG) had decreased by 12% on average (p < 0.01) by the time of the second follow-up, that is, after 14-30 months of zidovudine treatment. A decrease in neopterin during antiretroviral treatment correlated with an increase in tryptophan (p < 0.01). The data suggest that an association between decreased immune stimulation and reduced tryptophan degradation in patients treated with zidovudine.