Chelonian sea turtles use their lung as a buoyancy organ and as the major oxygen store when diving, and hence, buoyancy regulation and oxygen consumption can be expected to interact. The buoyancy of seven juvenile loggerhead turtles, Caretta caretta, was determined by measuring directly their underwater weight (M uw) while they were resting on a freely suspended weighing platform at a depth of 80 cm. Underwater weight was recorded continuously for 2 days for each turtle, followed by another 2 days measurements during which the turtles carried lead weights attached to their carapace, and finally, a last day of measurement after the weights had been removed. Total duration of resting dives (t r), buoyancy (F B), total resulting force acting on the resting turtle (F res) and body volumes were derived from the M uw data. Turtles were slightly negatively buoyant when resting (F res=−0.2943 to −0.981 N kg−1) and M uw increased significantly throughout each apnoeic period, meaning that the turtles progressively lost buoyancy. Pulmonary gas loss was calculated from the rate at which buoyancy decreased, which was significantly slower during the first half of the dive than during the second half of the dive. Resting oxygen consumption rates (V O2 ) were calculated from these data assuming that the pulmonary gas loss represents oxygen consumption from the lung. The V O2 obtained in this way (1.69–4.86 ml O2 min−1) corresponded well with other published and V O2 measured previously on loggerhead turtles in the same facility. Using oxygen from the lung affects buoyancy and may have impacts on the diving behaviour.