1. We have studied the effect of the anti-inflammatory anti-asthma drug, nedocromil sodium, on down-regulation of pulmonary β-adrenoceptors in guinea-pig lung. 2. Incubation of minced lung with isoprenaline (10 μmol/l) resulted in a reduction in maximum binding capacity of [ 125 I]iodocyanopindolol to lung membranes from 246 ± 4 to 169 ± 6 fmol/mg of protein (mean ± sem , P < 0.01, n = 18). 3. Nedocromil sodium, which had no direct effect on [ 125 I]iodocyanopindolol binding, prevented isoprenaline-induced down-regulation, giving complete protection at a dose of 100 μmol/l. 4. The mechanism of this effect is not certain, but nedocromil sodium may interfere with the internalization of β-adrenoceptors in pulmonary parenchymal cells. This may have some therapeutic relevance.