['Several experimental and clinical studies have demonstrated a direct bronchoconstrictor effect of opioids on smooth bronchial musculature following iv administration. The aim of this study was to evaluate the effects of alfentanil on respiratory system mechanics in a group of ASA I patients ventilated mechanically during general anesthesia.', 'Twenty consecutive ASA I patients (ten men and ten women) scheduled for general surgery interventions were studied (mean age 45.4 +/- 9.9 yr, mean weight 61.9 +/- 6.7 kg). Exclusion criteria were a history of chronic obstructive pulmonary disease, asthma or other pulmonary disease, atopy, wheezes, smoking and age below 18 yr. Subjects were randomly divided in two groups: Group A, receiving alfentanil at a 15 microg x kg(-1) dose and Group B receiving alfentanil at a 30 microg x kg(-1) dose. Respiratory mechanic variables were acquired at baseline (T0) and after three, ten and 15 min (T1, T2 and T3, respectively). We compared the basal values to the values measured at each time interval; basal values, prior to drug administration, served as control for each patient. P values < 0.05 were considered statistically significant.', 'We did not observe significant differences in respiratory mechanic variables after the administration of alfentanil, 15 and 30 microg x kg(-1). More specifically, respiratory system compliance and the different subcomponents of respiratory system resistances (i.e., maximum, minimum and delta resistance of respiratory system) were within normal limits and did not vary after alfentanil administration.'] Does alfentanil increase resistance of the respiratory system in ASA I patients ventilated mechanically during general anesthesia?