Many methods are available for digital computation of the impulse response from indicator dilution measurements representing input and output signals. In all instances, the only criterion for validity of the computation is comparison of the reconvolution of the computed impulse response and the input with the actual output. In this paper, a model mathematical system was constructed with a known impulse response; noise and time variation could be introduced independently or simultaneously in the input and the output data. Six methods for digital computation of the impulse response were applied to data from this system and to actual dye dilution data. Precision of reconvolution did not assure that the computed response would resemble the actual response of the system. Some numerical considerations also significantly affected the digital computation of a valid response.