Abstract In recent years, research on the pharmacokinetics of medicinal substances has become of exceptional importance both in terms of increasing the efficiency and safety of drug treatment of various diseases. In the development of a drug, the determining factor becomes the need to ensure a uniform release of the drug from the form-forming component, which is established in in vitro and in vivo experiments. Pharmacokinetic studies of the non-steroidal anti-inflammatory drug phenylglyoxylic acid derivative fensulcal in rectal dosage form suppositories have been studied. The studies were carried out according to the developed methods for the detection and release of fensulcal in biological fluid. According to the dynamics of release in in vivo experiments , it was found that the maximum concentration of fensulcal in the blood was reached after 1 hour. The study of the parameters in the in vitro and in vivo experiments led to the need to calculate the degree of their correlation. A high degree of correlation was established. The results on the value of the correlation coefficient indicating the existence of a direct relationship between the intensity of release in experiments in vitro and its entry into the blood in vivo after rectal administration of suppositories to animals. To assess the effect of a drug and its routes of administration on the rate of absorption of drugs and bioavailability, formalization of their pharmacokinetics was carried out in the form of a one- or two- part model . To assess the pharmacokinetics of Fensulkal suppositories, model-independent parameters were calculated taking into account a 2-chamber model. The results indicate that the maximum amount of fensulcal in the blood is observed 1 hour after administration (P <0.05 ). After reaching the maximum concentration, fensulcal is distributed in organs and tissues at a speed of 504.03 / min. Elimination of the drug proceeded at a lower rate, K β , h -1 = 0.0182. The constants of the fensulcal transition from the central to the peripheral chamber and vice versa are calculated. From the presented data follows K 21 >> K 12 , which indicates the absence of drug accumulation in the tissues.