Some alkylxanthines which are known as adenosine receptor antagonists or bronchodilators and phosphodiesterase (PDE) inhibitors have been studied using geometry optimisation by the AM1 molecular orbital method and electric field mapping using Mulliken charges. The possible atomic sites of these molecules which may be involved in interaction with the adenosine receptor and with the receptor related to bronchodilatory activity have been identified, and thus structure-activity relationships have been established for them. PDE inhibitor and bronchodilatory activities seem to have similar molecular structural requirements.