Juvenile chinook salmonOncorhynchus tshawytschawere fed erythromycin thiocyanate for 21 d at a daily therapeutic dosage of either 0.05 or 0.1 g erythromycin/kg body weight. We analyzed samples of liver, spleen, kidney, red muscle, white muscle, and plasma removed from fish at intervals during and after drug administration to determine the concentration of erythromycin present. On all but one sampling date, the kidney had the highest mean concentration (weight of drug per unit of tissue weight) among the tissues sampled. The mean concentrations in all tissues and plasma offish fed at the daily dosage of 0.05 g/kg did not differ significantly over time while the drug was fed. However, erythromycin accumulated over time in the liver, spleen, kidney, red muscle, and white muscle offish fed at 0.1 g/kg; concentrations of the drug peaked on the last day of antibiotic administration. Doubling the dosage from 0.05 to 0.1 g/kg resulted in a 4–10-fold increase in erythromycin concentration in tissues, and a twofold increase in plasma on the last day of feeding. The patterns of depletion of erythromycin in tissues and plasma of chinook salmon fed either drug-treated ration were similar. Small quantities of erythromycin were detectable in the liver and kidney 19 d after drug administration ceased, but not in the muscle.