Measurement of cerebral blood flow (CBF) using a 133Xe inhalation technique requires monitoring the clearance rate of the radioisotope from the head using externally situated detectors and also from arterial blood using a detector to monitor the expired air activity. The end-tidal concentration function is assumed to be proportional to the arterial concentration function and this has to be deconvoluted from the clearance functions obtained from the head to enable CBF to be calculated. A digital computer is generally considered to be essential for this. In this paper a procedure using three-dimensional nomograms is derived and tested. It is shown that results obtained using the nomograms do not differ significantly from results obtained using a full deconvolution procedure, and so it is not essential to use a computer to calculate CBF by the inhalation technique.