Aqueous-ethanolic extracts of Fraxinus excelsior, Populus, tremula and Solidago virgaurea in a combination of 1: 3: 1 (v/v/v) are the components of the plant drug Phytodolor N (abbreviated as PD), which exhibits antipyretic, analgesic and antirheumatic activity. Similar to a broad variety of synthetic non-steroidal anti-inflammatories the mentioned plant extracts inhibit dihydrofolate reductase. The following concentrations as percentage in the test volumes represent the individual I50-values: F. excelsior = 0.26% (v/v); P. tremula = 0.46% (v/v) and S. virgaurea = 0.6% (v/v). The combined extracts in PD exhibit an I50 at 0.3% (v/v). Testing the activity of the water-soluble compounds of corresponding dry extracts, the activity of F. excelsior with an apparent I50-value of 0.008% (w/v) by far dominates the inhibitory overall effect of the combination (I50 = 0.014%, w/v).