A simple method of precisely administering a compact gas bolus during selectable portions of the inspiratory cycle was developed to examine certain fundamental attributes of lung physiology and ventilation. The method allows for a small, well-defined bolus to be delivered during different phases of inspiration in a precise fashion. A pneumatically driven three-way switching valve is synchronized to the breathing pattern and controls the interjection of radioxenon boli in the inspiratory stream, spiking a small portion of the inhaled breath. The 133Xe dispersion and redispersion in the lungs is mapped and the clearance followed. Regional differences are analysed to determine how ventilation might influence the development of lung diseases due to inhaled toxins. Scintillation imaging at 20 frames s-1 allows near real-time characterization of the ventilation distribution. Novelty comes from the application of fast data gathering, the flow through valve design for delivery of tightly compacted boli and from tracing different fractions of the breath. The procedure involves low radiation exposure. An apparatus and method for acquiring data on lung filling and emptying not previously available is described. Potential applications are identified which may allow the re-examination of orthodox lung physiology.