Dibutyryl cyclic AMP (DB-cAMP, 0.1 and 1.0 mg/kg) and the purine-2-receptor agonist methyl-thio-ATP (MT-ATP mg/kg) given by intratracheal (i.t.) administration to rats two hr before bronchoalveolar lavage (BAL) increased iNOS mRNA to be equal to or greater than that produced by i.t. LPS, without eliciting neutrophil infiltration into the alveolar space or the upregulation of tumor necrosis factor alpha (TNF alpha). Translation of DB-cAMP and MT-ATP stimulated iNOS mRNA into protein and activation of iNOS to produce RNI was slower than that resulting from LPS-stimulated iNOS mRNA. Diethyldithiocarbamate (5 mg/kg, i.t.) a sequestrant of reactive oxygen intermediates and an inhibitor of NFkappaB attenuated LPS-induced upregulation of iNOS mRNA without affecting that produced by DB-cAMP or MT-ATP. We conclude that an LPS and cytokine-independent pathway of transcription of iNOS mRNA exists in vivo, which can be directly activated by DB-cAMP and purine-2 receptor stimulation. It is possible that the increase in iNOS found in asthmatic patients and those with other diseases that are treated with drugs which affect the cAMP and purine systems may be iatrogenic rather than pathogenetic in origin.