A non-genomic antisecretory role for dexamethasone at low concentrations (0.1 nM to1 µM) is described in monolayers of human bronchial epithelial cells in primary culture and in a continuous cell line (16HBE14o- cells). Dexamethasone produced a rapid decrease of [Ca²⁺]_i (measured with fura-2 spectrofluorescence) to a new steady-state concentration. After 15 min exposure to dexamethasone (1 nM), [Ca²⁺]_i was reduced by 32 ± 11 nM (n = 7, P < 0.0001) from a basal value of 213 ± 36 nM (n = 7). We have shown previously that aldosterone (1 nM) also produces a rapid fall in [Ca²⁺]_i; however, after the decrease in [Ca²⁺]_i induced by dexamethasone, subsequent addition of aldosterone did not produced any further lowering of [Ca²⁺]_i. The rapid response to dexamethasone was insensitive to pretreatment with cycloheximide and unaffected by the glucocorticoid type II and mineralocorticoid receptor antagonists RU486 and spironolactone, respectively. The rapid [Ca²⁺]_i decrease induced by dexamethasone was inhibited by the Ca²⁺-ATPase pump inhibitor thapsigargin (1 µM), the adenylate cyclase inhibitor MDL hydrochloride (500 µM) and the protein kinase A inhibitor Rp-adenosine 3â,5â-cyclic monophosphorothioate (200 µM), but was not affected by the protein kinase C inhibitor, chelerythrine chloride (0.1 µM). Treatment of 16HBE14o- cell monolayers with dexamethasone (1 nM) inhibited the large and transient [Ca²⁺]_i increase induced by apical exposure to ATP (10^-4 M). Dexamethasone (1 nM) also reduced by 30 % the Ca²⁺-dependant Cl^- secretion induced by apical exposure to ATP (measured as the Cl^--sensitive short-circuit current across monolayers mounted in Ussing chambers). Our results demonstrate, for the first time, that dexamethasone at low concentrations inhibits Cl^- secretion in human bronchial epithelial cells. The rapid inhibition of Cl^- secretion induced by the synthetic glucocorticoid is associated with a rapid decrease in [Ca²⁺]_i via a non-genomic mechanism that does not involve the classical glucocorticoid or mineralocorticoid receptor. Rather, it is a result of rapid non-genomic stimulation of thapsigargin-sensitive Ca²⁺-ATPase, via adenylate cyclase and protein kinase A signalling.