Centaurea solstitialis (yellow star thistle) has been proven to cause equine nigropallidal encephalomalacia in horses. Over the last fifty years, nigropallidal encephalomalacia has been of interest to human medicine due to the possible connection with Parkinson's disease. Previous studies indicated the presence of neurotoxic nitrogenous compounds in polar extracts of the plant. In order to give a more detailed description of the nitrogen-containing fraction of C. solstitialis, various samples were collected at different development stages. Different aliquots of the same aqueous extract were directly derivatized with o-phthaldialdehyde and dansyl chloride and analyzed separately by reversed-phase HPLC. A complete profile of the free nitrogenous fraction of C. solstitialis was given and results obtained with the two derivatization procedures were compared. No particularly high level of free aspartic and glutamic acids, two potent neuroexcitotoxic amino acids, were found in polar extracts of the plant. Tyramine resulted to be the most important biologically active amine present in C. solstitialis (with a mean concentration of 2.0 mg/100 g of dry weight).