We presented a plethysmograph built from an old pressure transducer associated with a mercury column. The paw under study was immersed into a mercury vessel for making the measurements. The resultant upward force exerted by the paw was equal to the weight of the fluid that the paw displaced. Since the force exerted equals to the product of the pressure times the area, and the area of the pressure sensor was constant, then the force was directly proportional to the pressure.  The accuracy of the measured volume was dependent on the precision of the sensor. In this case, the sensitivity was 50 to 300 µV per Volt, per 10 mm of mercury. This plethysmograph was tested by immersing objects with different volumes and shapes. Results showed a very good linearity. The values recorded were coupled to a PowerLab model 4/30, allowed us to digitalize the values obtained.

Key words: Plethysmometer, Pressure transducer, Rat’s Paw Volume