Plasma oxytetracycline concentrations were studied in four healthy beagles after oral administration of four different commercial preparations of oxytetracycline dihydrate tablets. Although no statistically significant differences were found in the biological availability between the four tablets, assessed by the peak plasma level and the area under the plasma concentration—time curve, there was great variation in the plasma concentrations of oxytetracycline achieved in the sixteen experiments. This variation was not related to thein vitrodissolution properties of the tablets. As the amount of oxytetracycline dihydrate administered in die experiments was almost the maximum recommended dose and the mean peak plasma concentrations were either below or at the lower end of the therapeutic concentration range, it is suggested that higher oral doses than those officially recommended are required to attain effective plasma level