1. The mechanisms underlying control of cyclic AMP responses to isoprenaline were studied in primary cultures of human airway smooth muscle cells. In these cells, isoprenaline induced concentration-related cyclic AMP formation via beta 2-adrenoceptor stimulation. 2. Prior incubation of cells with varying concentrations of isoprenaline (1-16 h), forskolin, prostaglandin E2 or a stable analogue of cyclic AMP all produced concentration-related desensitization of cyclic AMP responses to subsequent challenge with isoprenaline (maximum reduction with 1 mumol/l isoprenaline, 85% after 16 h). The desensitization induced over 2 h (44%) by a concentration of prostaglandin E2 which gave a similar rise in cyclic AMP levels to 1 mumol/l isoprenaline was significantly less (P < 0.05) than the desensitization (62%) induced over 2 h by 1 mumol/l isoprenaline itself. 3. Isoprenaline-induced desensitization of beta 2-adrenoceptor-induced cyclic AMP formation was insensitive to prior exposure of cells to dexamethasone. 4. These findings suggest that isoprenaline-induced desensitization of beta 2-adrenoceptor-induced cyclic AMP formation in primary cultures of human airway smooth muscle cells is mediated through both a cyclic AMP-dependent and probably an additional cyclic AMP-independent pathway, and that these pathways are insensitive to inhibition by glucocorticoids.