In two healthy male subjects the disposition of deuterated N-(2H3) acetyl-5-aminosalicylic acid (d3-ac-5-AS) was investigated after a single rectal dose of 500 mg d3-ac-5-AS. Urine and plasma were analysed by h.p.l.c. and gas chromatography mass-spectrometry. Peak concentrations of around 0.5 microgram/ml occurred within 6 h and plasma concentrations declined thereafter with a half-life (t1/2) of about 6 h which was confirmed by urinary excretion data. Renal clearance of d3-ac-5-AS ranged between 200 and 300 ml/min and only 4.4-11.2% of the dose could be recovered in the 48 h urine. Since no undeuterated ac-5-AS could be detected in any of the plasma and urine samples an irreversible acetylation of 5-AS is assumed in man.