This paper describes a new way of expressing local ventilation using a scintillation gamma camera interfaced to a digital computer. From the ratio of the first to the zeroth moment of the lung clearance curve for 13N gas, a mean transit time is calculated and displayed for different regions of the lung. Lung clearance curves following intravenously administered 13N solutions, or after equilibration with 13N gas, are measured during normal breathing. Describing regional ventilation by mean transit time during clearance uses all the counts in the lung clearance curve and the improvement in statistical accuracy compared with the height over area method using a Stewart-Hamilton equation is significant. In normal subjects the average mean transit time for clearance from the total lung field is t = (61 +/- 4) s, from the upper zone tuz = (64 +/- 4) s and from the lower zone tlz = (54 +/- 5) s. These results show uneven ventilation between regions of the normal lung in general agreement with that described by previous authors. In addition, the functional image gives the mean transit time for each cell of the 64 X 64 matrix of the computer display. The fractional error in mean transit time is 3% for an area of 2-2 cm2 corresponding to 12 cells in the display. This display provides a sensitive method, functionally and spatially, of investigating local differences in ventilation, including vertical and horizontal gradients in narrow strips of lung.