An experimental study has been conducted into the variability of respiratory muscles efficiency on ten adult subjects (males). According to pulmonary function test, all volunteers were found to be normal, from a respiratory aspect. Following this test, and using two spirometers for inspiratory and expiratory processes and a pressure transducer, each of static respiratory compliance (Cstrs), airway resistance (Raw) and instant-by instant muscles pressure (Pmus) as well as change in volume (AV), and thus work of breathing were measured throughout a (1/2) minute breathing perroti with presence or (and) absence of external resistances (glass tubes connected to the respiratory circuit), and external tension forces (springs) on the subject's chest wall. Coincided with these measurements, (O2) consumption ' was measured, thus energy consumed was found.The study also generates the cost of breathing from the latter value of total energy consumed for the (1/2) minute breathing period: Finally, the efficiency of the respiratory muscle was normally calculated.The obtained results suggest that the response of the respiratory muscles in ten subjects is almost, and to a high extent, the same. This is elucidated by having increased output and decreased input with increase in external loading (externally connected resistance) until a critical resistance is reached, i.e. efficiency of the respiratory muscles increased with increase in external loading. Beyond the critical resistance, energy consumed over ( 1 / 2 ) minute breathing period increased and both work done and theefficiency decline gradually until they diminish to negligible levels indicating that at the same resistance the respiratory muscles in all subjects volunteered refrain from the required work to admit a sufficient volume of air necessary to maintain body needs for Oxygen. Reciprocal behavior of respiratory muscles efficiency and other parameters has been found with increments in external tension force applied on the chest wall, i.e. the efficiency decreases with increment in the external tension force applied on the chest wall, up to a critical tension force where there is a noticeable increase in the efficiency up to the maximum tension force at which the largest admission of air takes place for the compensation of the shortage of necessary Oxygen supply