1. Average intracellular calcium concentration ([Ca2+]i) and ciliary beat frequency (CBF) were simultaneously measured in rabbit airway ciliated cells in order to elucidate the molecular events that lead to ciliary activation by purinergic stimulation. 2. Extracellular ATP and extracellular UTP caused a rapid increase in both [Ca2+]i and CBF. These effects were practically abolished by a phospholipase C inhibitor (U-73122) or by suramin. 3. The effects of extracellular ATP were not altered: when protein kinase C (PKC) was inhibited by either GF 109203X or chelerythrine chloride, or when protein kinase A (PKA) was inhibited by RP-adenosine 3', 5'-cyclic monophosphothioate triethylamine (Rp-cAMPS). 4. Activation of PKC by phorbol 12-myristate, 13-acetate (TPA) had little effect on CBF or on [Ca2+]i, while activation of PKA by forskolin or by dibutyryl-cAMP led to a small rise in CBF without affecting [Ca2+]i. 5. Direct activation of protein kinase G (PKG) with dibutyryl-cGMP had a negligible effect on CBF when [Ca2+]i was at basal level. However, dibutyryl-cGMP strongly elevated CBF when [Ca2+]i was elevated either by extracellular ATP or by ionomycin. 6. The findings suggest that the initial rise in [Ca2+]i induced by extracellular ATP activates the NO pathway, thus leading to PKG activation. In the continuous presence of elevated [Ca2+]i the stimulated PKG then induces a robust enhancement in CBF. In parallel, activated PKG plays a central role in Ca2+ influx via a still unidentified mechanism, and thus, through positive feedback, maintains CBF close to its maximal level in the continuous presence of ATP.