A morphometric analysis was made of alterations in serous cell structure induced by adrenergic and cholinergic agonists. Ferret tracheal rings were exposed for 30 min in vitro to one of the following agonists: phenylephrine, terbutaline, or methacholine (all at 10(-5) M). Controls were incubated similarly in medium containing no drugs or medium containing both the agonist and an excess of the appropriate antagonist (phentolamine, propranolol or atropine, all at 10(-4) M). Electron microscopic observation and stereological analysis of the incubated samples revealed that the volume density of serous cell granules in controls (0.30 +/- 0.02, mean +/- SE, n = 4) was significantly reduced by phenylephrine (0.19 +/- 0.03, n = 4) and methacholine (0.17 +/- 0.01, n = 4), but not by terbutaline (0.27 +/- 0.04, n = 4). The presence of antagonists in the medium prevented the observed changes (phenylephrine/phentolamine: 0.29 +/- 0.03, n = 3 and methacholine/atropine: 0.33 +/- 0.06, n = 3). In addition, the volume density of intracellular vacuoles in controls (0.02 +/- 0.05, n = 4) was increased in response to methacholine stimulation (0.12 +/- 0.05, n = 4), but not in response to the other agonists. This effect was blocked by atropine (0.01 +/- 0.00, n = 3). We conclude that serous-cell granules are discharged by both alpha-adrenergic and cholinergic, but not beta-adrenergic stimulation. In addition, cholinergic stimulation evokes the formation of intracellular vacuoles, a possible indication of active ion and water transport.