The pharmacokinetics of prednisolone have been studied in asthmatic patients following intravenous injection at three different doses and in normal volunteers at five oral doses. Plasma prednisolone concentrations were measured by radioimmunoassay. With increasing dose there is an increase in the apparent volume of distribution, plasma clearance and half life. The relationship between area under the plasma concentration time curve, maximum concentration and dose is linear but the regression lines do not pass through the origin. These findings following oral and intravenous administration confirm that prednisolone shows non-linear kinetics.