The response of HT29 cells to alpha 2-adrenoceptor agonists was investigated by X-ray microanalysis. Treatment of the cells with 10 microM UK14,304, an alpha 2-agonist, resulted in a rise of intracellular Cl and Na, suggesting an antisecretory effect. When the cells were exposed to yohimbine or RX821002, both of which are alpha 2-antagonists, the former had no effect, whereas the latter produced a minor decrease of the cellular Na, Cl and K concentrations. However, when the cells were pretreated with yohimbine, UK14,304 induced a transient change in ion concentrations, which consisted of a rapid increase of intracellular Cl, Na and a decrease of K at 1 and 5 minutes. Concurrently, a marked increase in intracellular Ca was found. This transient change could also be induced by different doses of UK14,304 (100 nM or 10 microM) after preincubation with yohimbine (0.3 microM or 3 microM). In contrast, UK14,304 did not elicit a similar response after pretreatment of the HT29 cells with RX821002. Also, yohimbine did not affect the ion content of the cells after pretreatment with UK14,304. These results indicate that yohimbine may have a specific effect by which the cells are sensitized to the ion transient induced by UK14,304.