The disposition of 5-aminosalicylic acid (5-AS), the therapeutically active metabolite of sulphasalazine (SZ), has been studied in patients with active inflammatory bowel disease, in patients with biliary tract disease and post-operative T-tube drainage, and in healthy volunteers. Subjects were treated 3 times a day either with 5-AS 0.5 g suppositories and a slow-release preparation or with SZ 1 g tid (equivalent to 5-AS 1.14 g/day). Plasma and urine concentrations of 5-AS and its acetylated major metabolite (AcAS) were monitored during one dosing interval. In a cross-over trial in 5 patients with ulcerative colitis no difference, was found in the dose-corrected mean (+/- SD) steady state plasma levels (Css) of 5-AS and AcAS between treatment with 5-AS suppositories (0.10 +/- 0.07 and 0.50 +/- 0.20 micrograms/ml, respectively) and SZ (0.12 +/- 0.14 and 0.67 +/- 0.14 micrograms/ml, respectively). Urinary excretion of total AS (5-AS + AcAS), too, was similar (192 +/- 70 and 179 +/- 79 mg/day) with both forms of treatment. The oral slow-release form of 5-AS produced slightly higher Css in 5 patients with Crohn's disease (5-AS 0.21 +/- 0.22 micrograms/ml; AcAS 0.83 +/- 0.40 micrograms/ml) and in 5 healthy volunteers (5-AS 0.28 +/- 0.14 micrograms/ml; AcAS 1.10 +/- 0.43 micrograms/ml). Urinary recovery of total AS averaged 20 +/- 6% (patients) and 27 +/- 10% (volunteers).(ABSTRACT TRUNCATED AT 250 WORDS)