1. Biliary excretion of sulphobromophthalein (BSP) was studied in male rats after oral administration of tilorone HCl, an anti-viral and anti-tumour agent. 2. The stimulation of bile flow by BSP was decreased 1 h after tilorone administration; this decrease was no longer apparent 24 h later but a fall of basal bile flow was noted at that time. No changes in bile flow were observed 7 days after this single dose. 3. Repeated dosing with tilorone produced a dose-dependent increase in hepatic concentration of unchanged and metabolized tilorone. This was associated with a dose-dependent decrease of bile formation and BSP excretion. Bile acid excretion was depressed as well. 4. BSP metabolism was not significantly affected by tilorone administration.