Mean residence time in the body may be determined by noncompartmental methods following any type of input process into the sampled compartment. Mean residence time in the central compartment can be determined following an intravenous bolus dose, as it requires calculation of the concentration at zero time. For any other input process the mean residence time in the central compartment can be calculated from the elimination rate constant from the central compartment if one accepts the same restrictions used to calculate mean residence time in the body.