The aim of this study was to test whether formate is formed during alpha-oxidation of phytanic acid in humans. To a healthy volunteer, [1-13C]phytanic acid was given as an oral substrate in a dose of 15 mg/kg body weight, after which plasma, urine and breath air samples were collected during 35 h. The plasma concentrations of [1-13C]-phytanic acid, 2-hydroxy[1-13C]phytanic acid, pristanic acid and [13C]formate were analysed. The [1-13C]phytanic acid concentration increased within 5-7 h to 105 mumol/l, then decreased. Formation of 2-hydroxy[1-13C]phytanic acid increased during the first 11 h after which it decreased during the next 20 h. Pristanic acid increased slightly during the test. In breath air, 13CO2 enrichment was measured, showing a cumulative output of ca. 30% of the ingested dose after 35 h. In both urine and plasma, enrichment of [13C]formate, higher than that of 13CO2 was demonstrated. These findings show that formate is a decarboxylation product in the alpha-oxidation of phytanic acid in vivo.