Application of 5-aminolevulinic acid (ALA) and its esters for treatment of cancer and other disorders became a rapidly developing branch of photodynamic diagnosis and therapy. For clinical use, solutions of ALA (must be buffered to physiological pH. Unfortunately, in such conditions they are unstable. Two molecules of ALA condense and 2.5-(beta-carboxyethyl)-dihydropyrazine and 2.5-(beta-carboxyethyl)-pyrazine are formed. Although, numerous eleborations relate to stability of ALA alone very little or none is known about the stability of ester derivatives of ALA. The present investigations have comprised the stability of ALA and its esters at 37 degrees C in respect of pH. concentration and time of reaction. The studies showed that ALA esters alike ALA undergo dimerisation at pH near 5. Moreover, it is possible that at pH>5.5 hydrolysis of the esters occurs, but this elaboration do not comprise explanation of this phenomenon.