Olsalazine sodium (5,5'-azodisalicylic acid (OLZ] was given to eight healthy volunteers as a 10 mg i.v. bolus dose and as a 1 g oral dose with and without food. To five fasting participants single oral doses of 2 g and 4 g were given. Blood and urine were collected during three weeks after each dose and were assayed for OLZ, a conjugate identified as a sulphate of OLZ (OLZs), 5-aminosalicylic acid (5-ASA), and N-acetyl-5-aminosalicylic acid (ac-5-ASA). The study showed that: 1. OLZ had a very short elimination half-life, mean 56 min. 2. OLZ was absorbed from the intestinal tract to a very small extent, as seen from the low systemic availability and low urinary excretion, 2.3% and 0.31% respectively, for a 1 g dose taken fasting. 3. OLZ was present in the serum partly as a conjugate, which was identified as an O-sulphate. Following the i.v. dose the serum half-life of the O-sulphate was estimated to be 7 days. 4. Food intake did not influence the systemic availability of OLZ and ac-5-ASA. 5. There was no dose-dependent increase of OLZ absorption with single doses up to 2 g, but a 4-g dose showed a more than two-fold increase in the individual peak serum concentration and in the systemic availability of OLZ. However, there was no significant increase in the mean residence time (MRT) for OLZ or in the serum concentration of either 5-ASA or ac-5-ASA at a dose of 4 g.