The mechanism by which leukotriene D4 (LTD4) induces bronchoconstriction in man is unclear. We have investigated this mechanism by examining the effect of nedocromil sodium and ipratropium bromide on LTD4-induced bronchoconstriction in six mild asthmatic patients. The bronchoconstrictor response to increasing concentrations of inhaled LTD4 (0.08 to 100 nmol) was assessed by measuring changes in respiratory resistance (Rrs) at 6 Hz by the forced oscillation method. Results were expressed as the provocative dose causing 100% increase in Rrs (PD100). Neither nedocromil (4 mg) nor ipratropium (0.8 mg) showed significant effect on LTD4-induced bronchoconstriction in asthmatic subjects. These results suggest that in asthmatics LTD4-induced bronchoconstriction is unlikely to involve vagal reflexes.