The rate aspect of bioavailability for pharmaceutical dosage forms in general and sustained-release preparations in particular can be quantified by an extension grade (EG), which is readily computed from steady-state data after administration of a test product and a suitable reference. Although the EG is unrelated to specific pharmacokinetic models or statistical moment analysis, interindividual differences of disposition kinetics are properly accounted for. It characterizes drug input in its entirety (i.e., the combination of drug release and absorption kinetics) and assumes values in the interval [0, 1]. Its properties resemble in several respects those of the area under the concentration versus time curve quotient F, which indicates the extent of bioavailability. The EG can be calculated from steady-state plasma levels and urinary excretion rates and from corresponding profiles observed after administration of single doses. It is easy to implement specific experimental designs for the determination of the EG.