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Five healthy horses were given a sulfadoxine/trimethoprim combination (Borgal, Hoechst AG) i.v. on day 1. The next ten days the horses got once a day a sulfadimethoxine/trimethoprim combination orally (Trafigal, Hoechst AG). The doses were given as recommended. One horse received no medicaments for control. On each horse six bronchoalveolar lavages were performed. Blood samples were taken to calculate blood levels and elimination half lives. To determine the amount of substances in lavage fluid and plasma the high performance liquid chromatography (HPLC) was used. Regularly low quantities of sulfonamides and trimethoprim were detected in lavage-samples. The mean plasma concentration (n = 4) of sulfadoxine and trimethoprim 30 min after i.v. administration was 71.6 and 1.13 micrograms/g respectively. 24 h after injection the sulfadoxine blood level was 3.0 micrograms/g, while trimethoprim was no longer detectable. The average elimination half lives of sulfadoxine and trimethoprim were 7.94 h and 1.35 h respectively. 8 h after oral application (n = 5) the highest mean sulfadimethoxine blood levels of 53.8 micrograms/g were measured. The elimination half life of sulfadimethoxine was 9.77 h. Two hours after feeding the drug the first blood samples were taken. They already contained the highest mean trimethoprim concentration of 0.32 microgram/g plasma.