SDZ MKS 492 (R(+)-(8-[( 1-(3,4-Dimethoxyphenyl)-2-hydroxyethyl)amino]-3,7-dihydro-7-(2- methoxyethyl)-1,3-dimethyl-1H-purine-2,6-dione) relaxes airway smooth muscle in vitro and impairs, or reverses, spasm of guinea-pig airways in vivo. Effects on bronchospasm in the guinea-pig may additionally include actions on airway hyperreactivity, since SDZ MKS 492 can inhibit the development, or expression, of airway hyperreactivity due to infusion of PAF, (+/-)isoprenaline, endotoxin or injection of immune complexes. In addition to influencing airway tone, inhaled SDZ MKS 492 diminishes the pulmonary accumulation of macrophages, eosinophils and neutrophils that follows inhalation of allergen by actively sensitized guinea-pigs. SDZ MKS 492 offers the prospect of a monotherapy for asthma, combining bronchodilator and anti-inflammatory activities with prophylactic efficacy.