A novel liquid chromatographic method for the determination of thiamine in plasma has been developed and has been used to study plasma thiamine concentrations after multiple dosage regimens for 11 days. The method involves purification, concentration and analytical separation of thiochrome on-line, using a switching column system. Ten healthy men were given 500 mg thiamine i.m. once a day (Group 1) and ten were given 250 mg p.o. every 12 h (Group 2). The times to reach steady state (7 and 5.6 days for Groups 1 and 2, respectively) were not different (P greater than 0.05). The mean elimination half-life was 1.8 days. The mean minimum steady-state concentration after the oral regimen (23 micrograms.l-1) was 78% of that after the intramuscular regime (29 micrograms.l-1).