Here, dA/d t is the rate of absorption or the rate of appearance of the substance in the blood, V is the apparent volume of distribution, C is the blood level in concentration units at time t , dC/dt is the slope of the blood level time curve at time t , and K is the first-order rate constant for loss of drug from the volume of distribution. The assumptions behind Eq. 1 are: (a) equilibrium between drug in blood and other fluids of distribution is maintained; (b ) V and K are constants, independent of time, over the time interval Eq. 1 is applied; and (c) the compound measured in the blood is the same compound as that absorbed (i .e. , A , in weight units, and C, in concentration units, refer to the same compound). Nelson ( 2 ) derived Eq. 2 which may be used for calculating the rate of absorption as a function of time from urinary excretion data