Among the tidal expiratory flow measurements that have been suggested as surrogate tests for airway obstruction, a short time to reach peak tidal expiratory flow (t(pef)) is the most widely used. Time to peak expiratory flow is most often expressed as the ratio between t(pef) and total expiratory time (t(e)). As te strictly depends inversely on respiratory rate (RR), we studied the hypothesis that an increase in RR (and a fall in t(e)) with the development of airway obstruction during methacholine or histamine challenge in infants could mask a decrease of t(pef) when expressed as t(pef)/t(e). Thirty-three infants (ages 6.5-23 months) with recurrent wheeze were studied during sedated sleep. Runs of tidal breathing and maximal expiratory flow at FRC (V(maxFRC)) measured by the squeeze technique were obtained before and after the challenge. All infants responded to the challenge: the median fall in P(tCO2) was 25%, and it was 43% in V(maxFRC). RR increased from a median value of 31.1 to a median of 35.1 breaths/min. Both t(pef) and t(e) were significantly shorter after the challenge (P < 0.001 and 0.004, respectively); however, the decrease in t(pef)/t(e) was not significant (P=0.081). The change in t(pef)/t(e) was positively correlated with the change in RR (r = 0.51, P = 0.003). To analyze better the effect of changes in RR on various indices, we divided the patients into two groups: in 17 subjects with a small increase in RR (< 10%), t(e) did not change significantly, while t(pef) and t(pef)/t(e) did; in 16 subjects with a more marked increase in RR (> or = 10%), the shortening of te masked the simultaneous shortening of t(pef), so that t(pef)/t(e) did not change. These data demonstrate that t(pef)/t(e) cannot be reliably used to evaluate changes in airway obstruction when concomitant changes in RR occur.