Equation expressing the build up and decay in concentration of a substance (usually in plasma) based on first-order uptake and elimination in a one-compartment model, having the form \[C = [fDk_{\rm{a}}/V(k_{\rm{a}} - k_{\rm{e}})] [\exp(-k_{\rm{e}}t) - \exp(-k_{\rm{a}}t)] \]where \(C\) is the concentration and \(D\) the dose of the substance, \(f\) the fraction absorbed, and \(V\) the volume of distribution. \(k_{\rm{a}}\) and \(k_{\rm{e}}\) are the first-order rate constants of uptake and elimination, respectively, and \(t\) is time.