We investigated the effects of temperature changes on the oxygen consumption rhythm in Japanese eels, Anguilla japonica, using an automatic intermittent flow respirometer (AIFR). The endogenous rhythm of the oxygen consumption rate (OCR) in the eels (n = 18; 44-74 cm, 145-690 g), freshly collected by bag net from estuaries, was nearly synchronous with the tidal pattern of the estuarine collection site. The magnitude of mean OCR (mOCR) of eels showed variable range of 82.2 - 116.5 ml $O_2;kg^{-1}ww ;h^{-1}$ under constant conditions. In case of increasing temperature from 25 to $38^{circ}C$, the OCR of eels exhibited a gradually increasing trend with a rhythmic pattern until $36^{circ}C$. Above $36^{circ}C$ the rhythms of the OCR dampened and the OCR decreased rapidly at around $36-37^{circ}C$. The OCR of the eels exhibited the maximum value at $38^{circ}C$, and then it sharply decreased. The results suggested that the critical thermal maximum (CTM) regarding the endogenous rhythms of the eels was at around $36-37^{circ}C$ when water temperature increased at $0.5^{circ}C$/14 h following the acclimation at $25^{circ}C$. In case of decreasing temperature ($0.5^{circ}C$/14 h) from 25 to $0^{circ}C$, the OCR of the eels displayed a abrupt decrease up to $23^{circ}C$, and between at 23 and $20^{circ}C$, there was an agitation which showed a slight increase in the OCR with a duration of 1-2 days. Below $9^{circ}C$, the OCR rhythm of the eels showed a constant state regardless of temperature decreasing. These results suggest that the Japanese eel has an upper incipient lethal temperature at $36^{circ}C$, with a lower thermal limit at $9^{circ}C$. The biochemical aspects of the eels influenced by water temperature need to be further studied.