In practical calculation of clearance from the disappearance rate, neither normal mixing nor a normal distribution space can be assumed as constants. Calculation of clearance via the injected pulse count is preferable to calculation via the activity eliminated in the urine, since the former is more accurate. 51Cr-EDTA is practically ideal for this clearance method. If indicated, this slope method can provide valuable information. However, because of its basic possibilities of error, it cannot replace the classic steady-state method.