Of the factors determining blood gases, only alveolar ventilation (A) is amenable to manipulation. However, current physiology text books neither describe how breath-by-breathA can be measured, nor how it can be precisely controlled in spontaneously breathing subjects. And such control must be effected independent of minute ventilation (E) and the pattern of breathing. Control ofA requires the deliberate partition of inhaled gas between the alveoli and the anatomical deadspace. This distribution is accomplished by sequential gas delivery (SGD): each breath consists of a chosen volume of ‘fresh’ gas followed by previously exhaled gas. Control ofA through SGD is a simple, inexpensive, yet powerful tool with many applications. Here we describe how to implement SGD, how it precisely controlsA, and consequently how it controls arterial blood gases.