The passive mechanical time constant (tau pass) of the respiratory system is relatively similar among newborn mammalian species, approximately 0.15-0.2 s. However, breathing rate (f) is higher in smaller species than larger species in order to accommodate the relatively larger metabolic demands. Since tidal volume per kilogram is an interspecies constant, in the fastest breathing species the short expiratory time should determine a substantial dynamic elevation of the functional residual capacity (FRC). We examined the possibility of a difference in expiratory time constant between dynamic and passive conditions by analyzing the expiratory flow pattern of nine newborn unanesthetized species during resting breathing. In most newborns the late portion of the expiratory flow-volume curve was linear, suggesting muscle relaxation. The slope of the curve, which represents the dynamic expiratory time constant of the respiratory system (tau exp), varied considerably among animals (from 0.1 to 0.7 s), being directly related to the inspiratory time and inversely proportional to f. In relatively slow-breathing newborns, such as infants and piglets, tau exp is longer than tau pass most likely due to an increase in the expiratory laryngeal resistance and FRC is substantially elevated. On the contrary, in the fastest breathing newborns (such as rats and mice) tau exp is similar or even less than tau pass, because at these high rates dynamic lung compliance is lower than its passive value and the dynamic elevation of FRC is small. In dynamic conditions, therefore, the product of tau exp and f is maintained within narrow limits.(ABSTRACT TRUNCATED AT 250 WORDS)