Single bronchiolar ciliary cells were isolated from rat lungs. The b2-adrenergic regulation of ciliary beat frequency (CBF) was studied using video-optical microscopy. Terbutaline (an ₂- adrenergic agonist) increased CBF in a dose-dependent manner, and it also decreased the volume of the ciliary cells. These terbutaline actions were inhibited by a PKA inhibitor (H-89) and mimicked by forskolin, IBMX and DBcAMP. Ion transport inhibitors were used to isosmotically manipulate volume of the terbutaline- stimulated bronchiolar ciliary cells. Amiloride (1 然) and bumetanide (20 然) potentiated cell shrinkage and the CBF increase, and they shifted the terbutaline dose-response curve to the lower- concentration side. Quinidine (500 然), in contrast, increased cell volume and suppressed the CBF increase. Moreover, a KCl solution containing amiloride (1 然) and strophanthidin (100 然) increased cell volume and suppressed the CBF increase, and then the subsequent removal of either amiloride or strophanthidin decreased cell volume and further increased CBF. NPPB (10 然) or glybenclamide (200 然) had no effects on the terbutaline actions. Thus, in terbutaline-stimulated ciliary cells, cell shrinkage enhances the CBF increase, in contrast, cell swelling suppresses it. However, the results of direct manupulation of cell volume by applying osmotic stresses (hyperosmotic shrinkage or hypo-osmotic swelling) were opposite to the findings of the isosmotic experiments: hypo-osmotic cell swelling enhanced the CBF increase, while isosmotic swelling suppressed it. These suggest that isosmotic and non-isosmotic volume changes of terbutaline-stimulated bronchiolar ciliary cells may trigger different signaling pathways. In conclusion, terbutaline increases CBF and decreases the volume of the rat bronchiolar ciliary cells via cAMP accumulation under an isosmotic condition, and the isosmotic cell shrinkage enhances the CBF increase by increasing cAMP sensitivity.