The present study examined the pharmacokinetics and metabolism of sulphamonomethoxine (SMM) in rainbow trout (Oncorhynchus mykiss) and yellowtail (Seriola quinqueradiata) after intravascular administration (100 mg kg(-1) body weight), Rainbow trout and yellowtail were kept in tanks with running water at 15.0 +/- 0.3 degrees C and running sea water at 21.3 +/- 0.2 degrees C, respectively. Serum concentrations of SMM were determined using high performance Liquid chromatography with direct injection. Serum concentrations of SMM in rainbow trout and yellowtail were best described by a two-compartment model. The calculated half-lives for the distribution phase and the elimination phase were 0.43 h and 30.9 h for rainbow trout, and 0.53 h and 5.8 h for yellowtail, respectively, The apparent volume of distribution (V-d) was larger in rainbow trout (V-d = 0.83 l kg(-1)) than in yellowtail (V-d = 0.56 l kg(-1)). Total body clearance was calculated as 18.5 ml kg(-1) h(-1) in rainbow trout and 66.7 ml kg(-1) h(-1) in yellowtail. The behaviour of N-4-acetyl metabolite, N-4-acetyIsulphamonomethoxine (AcSMM), in rainbow trout and yellowtail could be explained well by a one-compartment model. The formation rate constant of the metabolite and the elimination half-lives were 0.0694 h(-1) and 15.4 h for rainbow trout, and 0.1896 h(-1) and 8.1 h for yellowtail, respectively. Acetylation in rainbow trout and yellowtail was calculated to be 23% and 64%, respectively. The serum protein bindings in vivo of SMM and AcSMM were determined to be 6.4 +/- 2.3% and 9.5 +/- 3.6% for rainbow trout, and 5.8 +/- 1.7% and 6.3 +/- 2.3% for yellowtail, respectively. (C) 1997 Elsevier Science B.V.