To determine the role of M1 muscarinic receptors in the response of the pulmonary parenchyma to inhaled methacholine (MCh), 20 mongrel, out-bred puppies, 8-10 weeks of age were challenged following pretreatment with either saline (control), UH-AH37 (a combined M1 & M3 receptor blocker), or pirenzepine (a relatively selective M1 receptor blocker). In addition, eight fox hound-beagle puppies, born and raised in a clean animal house, were studied. Relatively selective doses of pirenzepine produced a dose-dependent shift to the right of the parenchymal dose-response curves (P = 0.031), with no effect on the airway dose-response curve (P = 0.102). The fox hound-beagle puppies showed less parenchymal response (P <0.0005), but equivalent airway response (P = 0.468), to MCh compared with the mongrel puppies. High doses of pirenzepine (10 000 mu g/kg) and UH-AH37 (3 mg/kg) markedly inhibited both the parenchymal and airway responses to MCh. Data from the present study demonstrate that: (1) while both the airway and pulmonary parenchyma respond to inhaled MCh, the mechanisms by which they respond differ; (2) stimulation of M1 subtype muscarinic receptors are responsible, at least partly, for the parenchymal response; and (3) experimental conditions, such as the breed and housing conditions of animals, may have major influences on the parenchymal response to inhalational challenge tests.