Olsalazine sodium (Dipentum*) has been used therapeutically against inflammatory bowel disease in human medicine as an alternative to sulphasalazine over the past 20 years. Bacteria in the colon split this prodrug into two molecules of the locally effective 5-aminosalicylic acid (5-ASA). Considering the potential therapeutic use in equine colitis, the pharmacokinetics of olsalazine (OLZ) after single oral administration to six horses at a dosage of 30 mg/kg was investigated. Plasma concentrations of OLZ, 5-ASA, and its main metabolite N-acetyl-5-aminosalicylic acid (Ac-5-ASA) were analysed by high-performance liquid chromatography methods. Evaluation of the plasma pharmacokinetics revealed a rapid, but low extent of absorption of OLZ (peak concentrations around 1 microg/mL at 0.5-1.5 h), and a delayed minimal absorption of 5-ASA (concentrations < 0.2 microg/mL, at 11-35 h), which is immediately metabolized to Ac-5-ASA. As indicators of the local availability in the colon, high faecal water concentrations of 5-ASA and Ac-5-ASA (mean C(max) about 300 and 130 microg/mL, respectively), but only traces of OLZ were found in faeces excreted 18-50 h after dosing. Of the administered OLZ dose 26% could be recovered from faeces, almost completely as 5-ASA and Ac-5-ASA. Routine clinical examination of the horses and assay of standard haematological and serum chemistry parameters before and after OLZ administration confirmed that a single dosage of 30 mg/kg was well tolerated. To estimate the systemic availability of 5-ASA liberated from OLZ, 5-ASA was administered i.v. at a dosage of 1.5 mg/kg to four horses and plasma concentrations of 5-ASA and Ac-5-ASA were determined. The pharmacokinetic evaluation showed a very low bioavailability of 2.4% for 5-ASA, released from orally administered OLZ. Furthermore, in an in vitro experiment, the metabolic transformation of 5-ASA to Ac-5-ASA mediated by bacteria in the caecal content of horses was determined at 38 degrees C for 31 h and compared with the metabolism data of the in vivo study. The markedly lower degree of acetylation in vitro supports the assumption that biotransformation of 5-ASA in vivo occurs not only by colonic bacteria, but also at other sites.