Various substances have been used to investigate physiological and physiopathological processes in animals. In this study, we investigated the effects of acetylsalicylic acid (ASA, aspirin) on the regeneration of actinotrichia, skeletal structures of the caudal fin of teleosts. Two groups of fish (Tilapia rendalli) were maintained in aquaria with dechlorinated water at 24oC, with one group being exposed to ASA (0.1 g/l) for 24 h. Thereafter ASA-treated and untreated (control) fishes were anesthetized and their tail fin amputated. After periods ranging from 4-12 days, the fishes were sacrified and the regenerating tissue was processed for light and transmission electron microscopy and picrosirius-hematoxylin staining. Control specimens showed normal regeneration of the actinotrichia, whereas all (except one) of the ASA-treated fishes showed no regeneration. The 20 ASA-treated fishes devoid of actinotrichia had varying degrees of caudal fin regeneration. These results indicate that, as in mammals, aspirin also affects biological processes in fish. Based on reports in the literature, we hypothesize that ASA interfered with the transcription of the fibroblast genes necessary for the synthesis of elastoidin, or altered the typical rapid turnover of this protein, thereby affecting regeneration of the actinotrichia. The use of ASA and other drugs to study the regeneration of actinotrichia could be a valuable approach for investigating cell-matrix interactions. This model could also be useful for evaluating the toxic effects of river pollution and chemical damping.