Prostaglandins (PG) are very potent mediators which can dilate or constrict the bronchi. In order to evaluate the role of PG in the homeostasis of bronchial tone, we studied the effects of mefenamic acid, a potent cyclooxygenase inhibitor, on the pulmonary function tests of 20 volunteers. The subjects were studied randomly in two sessions, one with a placebo and one with mefenamic acid. Tablets (drug or placebo) were taken for 2 days with the pulmonary function tests performed in the afternoon of the second day. Sessions were performed at least 2 days apart. Parameters measured, including forced vital capacity, forced expiratory volume in 1 s, expiratory flows, functional residual capacity and specific airway conductance (SGaw), were similar in both sessions. The SGaw, which was previously reported to increase with cyclooxygenase inhibitors, was 0.23 +/- 0.06 s-1 X cm H2O-1 in the placebo session and 0.24 +/- 0.06 in the mefenamic acid session (mean +/- SD). This study shows that cyclooxygenase inhibitors have no effect on airway tone and strongly suggests that endogenous PG do not participate in the homeostasis of normal bronchial tone.