The goal of the study was to evaluate Lippia sidoides essential oil as an anesthetic for the tropical fish pacu Piaractus mesopotamicus. The physiological and pharmacokinetic responses were evaluated during fish handling. Fish were exposed to different concentrations of the essential oil (20, 30, 40, 50, 60, and 70 mg L-1) to determine the induction times to anesthesia. The hematological parameters were determined in fish exposed to different concentrations of the essential oil (20 and 70 mg L-1), which were enough to induce light and deep anesthesia in 10 min. For pharmacokinetics purposes, blood samples were collected after exposure to the essential oil (70 mg L-1) for 10 min at different depuration times (0, 15, and 30 min, 1, 2, 4, 6, 8, 24, and 48 h). Thymol was the major essential oil compound in Lippia sidoides, at 61.12%. All physiological parameters returned to control values in 24 h. Distribution and elimination of the major compound in the blood plasma were fast, being 64.31% in 1 h. The distribution constant of the essential oil was 7.59 lg h-1 (k) in the organism. Therefore, when the plasmatic concentration of thymol was reduced, this constant decreased to 0.44 lg h-1 (k). The AUC value was 38.05 lg h-1 , the initial half-life during the distribution stage (T 1/2 a) was 0.09 h and the elimination half-life (T 1/2 b) was 1.59 h. Fish survival was 100% even 20 days after the experiments. Therefore, Lippia sidoides essential oil may be considered a safe anesthetic for tropical fish.