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Trimethoprim (TMP) was administered in combination with either sulphadiazine or sulphadimidine to broilers, and plasma concentrations were determined simultaneously by newly developed thin-layer and/or high-performance liquid-chromatographic procedures, which also allowed quantification of the N4-acetyl metabolites of the sulphonamides. After i.v. injection of TMP (20 mg/kg body wt) and sulphadiazine (100 mg/kg body wt), both compounds were rapidly eliminated from plasma with half-lives of 1 and 2.7 h, respectively. Apparent volumes of distribution (3.3 and 0.96 l/kg, respectively) indicated that the tissue distribution of TMP was more extensive than that of the sulphonamide. After oral administration of the same dosages, elimination appeared to be slower compared to the i.v. injection, but this was obviously related to delayed absorption. Bioavailability after oral administration was approximately 100% of sulphadiazine, but only about 60% for TMP. Oral dosing of TMP in combination with sulphadimidine yielded similar maximum plasma concentrations of both compounds to those obtained with the combination of TMP with sulphadiazine, but the plasma concentration decline of sulphadimidine appeared to be more rapid than that of sulphadiazine after oral administration. During prolonged administration of different dosages of TMP-sulphadiazine combinations via drinking water, only low plasma concentrations were attained by the recommended dosage of the combination. Up to 10-fold higher dosages were tolerated by the animals without side-effects. In view of the fact that the sensitivity of bacterial strains to TMP-sulphonamide combinations differs widely, the plasma concentrations determined in the present study during prolonged drinking-water medication with different dosages of a TMP-sulphadiazine combination can be used to select effective doses for treatment of different poultry diseases.

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