The objective of this study was to determine the pharmacokinetics of two orally administered doses of tramadol (5 and 10 mg/kg) and its major metabolite (O-desmethyltramadol) (M1) in loggerhead sea turtles (Caretta caretta). After oral administration, the half-life of tramadol administered at 5 and 10 mg/kg was 20.35 and 22.67 hr, whereas the half-life of M1 was 10.23 and 11.26 hr, respectively. The maximum concentration (C_max) for tramadol after oral administration at 5 mg/kg and 10 mg/kg was 373 and 719 ng/ml, whereas that of M1 was 655 and 1,376 ng/ml, respectively. Tramadol administered orally to loggerhead sea turtles at both dosages provided measurable plasma concentrations of tramadol and O-desmethyltramadol for several days with no adverse effects. Plasma concentrations of tramadol and O-desmethyltramadol remained ≥100 ng/ml for at least 48 and 72 hr when tramadol was administered at 10 mg/kg.