LTD4 was more potent in airway tissues from immature male and female guinea-pigs than in those from mature animals. The intrinsic activity of LTD4 relative to carbachol was greatest in tracheal tissues from immature animals. Indomethacin treatment reduced the potency of LTD4 and increased maximal contractile responses in most tissues. The reduced potency of LTD4 as a consequence of maturation may be significant for the decrease in airway reactivity seen in children at adolescence.