The tissue distribution and depletion of ormethoprim (OMP, 5 mg kg(-1): day-(1)) and sulphadimethoxine (SDM, 25 mg kg(-1) day(-1)) were studied in Atlantic salmon (Salmo salar) after oral administration of Romet(30) in feed for 5 consecutive days. The seawater temperature was 10.0+/-0.5 degrees C and the salinity 33 parts per thousand. The concentrations of the drugs in plasma and the tissues were determined by high performance liquid chromatography. The plasma and tissue levels of OMP and SDM reached steady state levels between 3 and 8 days following initiation of medication. The highest average concentration of OMP in plasma, muscle, liver and kidney were 1.50, 3.67, 9.10 and 166.0 mu g ml(-1) (g(-1)), respectively. The corresponding values for SDM were 14.30, 17.72, 7.42 and 6.80 mu g ml(-1) (g(-1)), respectively. The elimination half-lives (t(1/2)beta) for SDM in plasma, muscle liver, and kidney were 20, 19, 62 and 45 h, respectively. The corresponding values for OMP were 63, 143, 95 and 410 h for plasma, muscle, liver and kidney respectively. The mean ratios of OMP:SDM at steady state concentrations in the various organs were 1:10, 1:5, 1:0.8 and 1:0.06 for plasma, muscle, liver and kidney, respectively. The in vitro minimum inhibitory concentration values (MIG) for Romet(30) and various OMP: SDM ratios against selected strains of Aeromonas salmonicida were 1-2 mu g ml(-1) for Romet(30) and for the ratios of 1:5 and 1:1. For the OMP: SDM ratios of 1:0, 1:1O and 1:20, the MICs were 2-4 mu g ml(-1). (C) 1997 Elsevier Science B.V.