Pieces of liver, kidney and urinary bladder were fixed in 10% formalin. In order to study the velocity of the dehydration process tissue specimens of standardized size were rinsed and equilibrated with water containing 3H2O. The specimens were then put into vials with 100% ethanol or acetone; the vials were either shaken continuously or left stationary. The concentration of 3H in the dehydration medium was determined at frequent intervals. If the vials were shaken, steady concentrations of 3H were reached within about 50 min when ethanol was used, and about 30 min with acetone, indicating that dehydration was complete. With the vials left standing still the corresponding times exceeded 12 h. In other experiments dehydration was carried out with intermittent shaking in rising concentrations of ethanol or acetone; in these cases about 4 h were required.