Instruments were attached to loggerhead turtles, Caretta caretta, at Kamoda Point, Japan in 1989 and on the Senri-coast, Japan in 1991. We simultaneously obtained stomach temperatures, ambient water temperatures and diving depths from four free-ranging loggerhead turtles during the internesting periods, using small recorders. These data were analyzed to understand how body temperature was maintained. Ambient water temperatures changed coincidentally when turtles moved up or down, but a stable stomach temperatures was maintained despite water temperature fluctuations of ca. 20 to 90 min in duration. Loggerhead turtles also experienced water temperature fluctuations longer than 24 h. Stomach temperatures responded to those water temperature changes with a time lag of several hours (160, 170, 230 and 240 min). Stomach temperatures were higher than ambient water temperatures throughout the experimental periods. The median values of the thermal difference between stomach and water temperatures were 1.1, 1.3, 1.5 and 1.7°C. The higher stomach temperatures are thought to be caused by metabolic heat production.