A transcutaneous blood gas monitor is a non-invasive tool that is used clinically. Magnetic resonance imaging (MRI) of the rat has been performed for basic research of pathology and brain science. Non-invasive monitoring of blood gas is expected for such small animal experiments because of the need to minimize blood loss. We applied a commercially available transcutaneous blood gas monitor to rat experiments with MRI. The monitoring responses to rat arterial tension of carbon dioxide (paCO2) appeared to change exponentially with the time constant 1820±40 s, which was five times slower than that of man. The jump in paCO2, however, can be translated only by a differential slope of values monitored by tras cutaneous measurement of carbon dioxide (tcpCO2) at the timing of paCO2change without waiting for the saturated value of tcpCO2. The magnetic field of MRI did not affect blood gas monitoring and magnetic material in the blood gas sensor did not jeopardize the magnetic homogeneity of imaging. The transcutaneous blood gas monitor can be applied to MRI measurements with paCO2jumps.