The willow tree acute toxicity test was applied to determine the acute toxicity of the neutral and the ionic form of the antibiotic trimethoprim (TMP) to trees. The pKa of the weak base TMP is at 7.2, and the toxicity testing was done at low (pH 4.3), medium (pH 6.4) and high pH (pH 8.15). The test gave toxic effects, i.e. reduced transpiration at an external concentration of 100 mg/L TMP at medium and high pH, where the neutral form of TMP dominates. At an external concentration of 1,000 mg/L, also at low pH an effect occurred, but it was less pronounced than at medium pH.