ABSTRACT:The metabolism of L‐tryptophan (TRP) before and after an oral loading dose of 20 mg/Kg body weight was studied in three patients with actinic reticuloid (AR) and four healthy male volunteers. The preload mean plasma TRP concentrations and mean 24‐hour urinary excretions of TRP in the AR group were significantly lower than in the control group. After oral TRP administration, plasma TRP concentrations increased normally in AR, thereby excluding defective intestinal absorption. The mean preload and postload excretions of kynurenic acid (KA), xanthurenic acid (XA), 5‐hydroxyindoleacetic acid (5HIAA), and 3‐indoleacetic acid (3IAA) were not significantly different between the two groups. We conclude that KA, which has been shown to have phototoxic properties in vitro, is not altered primarily in AR, and that the reported abnormalities of TRP metabolism are most tikely the consequence of the severe photosensitivity in this d