Xenobiotics are foreign substances of anthropological origin and pollutants of the environment, where they end up due to ineffective removal from wastewater. The aim of this work is to determine the toxicity of xenobiotics before and after photolysis at acidic, neutral and alkaline pH-values (4, 7 and 10), in order to determine the ecotoxicity of the examined components, mixtures of their degradation/transformation products, as well as the examination of potentially negative effects of the pH-value. A low pH-value is possible in the environment due to acid rain, while a high value occurs in natural waters with an increased concentration of mineral substances (calcium, magnesium, potassium) as a result of rock deterioration. Photolysis often takes place in the environment due to the presence of sunlight, with possible effect in different media. It is a process that degrades a large number of chemicals and can lead to the formation of degradation/transformation products that are more toxic than the original components. Therefore, the task of this paper is to simulate the conditions of photolysis, which significantly affects the transformation of xenobiotics in the environment, and to determine their toxicity. The toxicity tests were applied on samples of pesticides (acetamiprid, clothianidin, thiacloprid) and antiparasitics (mebendazole, albendazole and febantel). Toxicity was measured in accordance with the HRN/EN ISO 11348-1:2000 norm using the luminescent bacterium Vibrio fischeri, photolysis was performed in a SuntestCPS+ device for 300 min, and sample analysis was performed in an HPLC-DAD chromatographic system. All samples (10 mg L^-1) showed toxicity under certain conditions. The measurement results showed an unexpectedly high toxicity after photolysis of the clothianidin sample at pH=10 conditions and a successful effect of photolysis on the degradation of thiacloprid, mebendazole and albendazole without increasing toxicity. The toxicity measured on febantel samples was caused by the photolysis of the hydrolytic product in an alkaline medium (pH=10). It is necessary to carry out more precise analytical analyzes in order to identify the degradation products formed during the photolysis process.