Introduction. For pharmaceutical development of new dosage forms, it is necessary to study the permeability of the active pharmaceutical ingredient (API) through various biological barriers of the human body. Determination of permeability is one of the key stages in selecting the dosage form for a drug under development. Permeability determines not only the route of administration, but also dosage, as well as the presence and composition of excipients. One of the most widely used approaches for permeability assessment at the preclinical stage is experimentation on Caco-2 cell models. Their use allows simulation of conditions close to those in the human gastrointestinal tract [1]. This is a relatively inexpensive and sufficiently effective method for determining API permeability. Aim. Experimental determination of the degree of intestinal permeability of the antiviral drug Riamilovir (Triazavirin®) using the Caco-2 cell line model (colorectal adenocarcinoma). Materials and methods. The experiment to determine the permeability of the API riamilovir (Triazavirin®), manufactured by Medsintez Plant LLC, was performed using the Caco-2 cell line model (colorectal adenocarcinoma), obtained from the cell culture collection of the Institute of Gene Biology of the Russian Academy of Sciences (IGB RAS, Moscow, Russia). Cells were cultured on a membrane for 21 days, after which a riamilovir solution at a concentration of 250 μM was added. After incubation, the drug concentration in the basolateral medium was quantitatively determined using HPLC. Results. Analysis of the obtained chromatograms revealed an average riamilovir concentration in the acceptor wells of 8,42 μmol/L. The key parameter — the apparent permeability coefficient (PAPP) from the apical to the basolateral layer — was calculated as 5,2 × 10⁻⁶ cm/s, indicating a moderate (20–70%) degree of drug permeability. Discussion. The apparent permeability value of riamilovir indicates moderate permeability of the drug, but extrapolation of the results to absorption in the human body requires caution due to possible presystemic metabolism and the characteristics of the Caco-2 model. To obtain data that are more accurate it is needed to carry out additional experiments (under different pH conditions). Conclusion. The results of the experiment indicate a moderate ability of riamilovir to penetrate the intestinal epithelium. However, in order to select the most optimal dosage form with higher bioavailability, further studies of permeability using other biological models and the development of methods to enhance absorption are required.