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 = (ƒDκ_a/V(κ_a - κ_e))(exp(-κ_et) - exp(-κ_at))

where C is the concentration and D the dose of the substance, ƒ the fraction absorbed, and V the volume of distribution. κ_a and κ_e are the first order rate constants of uptake and elimination, respectively, and t is time.