The urinary excretion of caffeine in humans was followed over a period of 36 h after the oral administration of ANIMINE, a formulation containing caffeine-alpha-naphthylacetate. The excretion of caffeine was not as markedly affected by the urinary pH as was found with stimulant amines. Excretion peaks were obtained 1-2 h after the ingestion and the total amount of unchanged caffeine excreted during 12 h varied from 0.57 to 1.51 per cent. The ingestion of the diuretics acetazolamide or furosemide 2 h after caffeine resulted in a urine-flow dependent and consequently increased caffeine excretion during 2-4 h post-diuretic. This increase paralleled the increase in urine volume resulting in no meaningful differences in caffeine concentration compared to normal conditions.