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The paper describes an application of system identification techniques to the estimation of the cardiac output and other cardiopulmonary quantities in human subjects. The method used requires continuous measurements of gas flow rates and gas concentration at the subject's mouth and involves the use of a lumped parameter model of the gas exchange processes. Particular emphasis is given to investigation of the identifiability and determinacy of the model and to the important question of the choice of input test signal. Estimation of the parameters of the model is by an extension of the maximum likelihood method of ¿¿str¿¿m and Bohlin. The paper includes a summary of some encouraging results which have been obtained using this approach.