The effect of 0.5-1.0 #M taxol, a potent promoter of microtubule polymerization in vitro, was studied on the secretory activity of chromaffin cells of the adrenal medulla. Taxol was found to have a dual effect: the long-term effect (after a 1-h incubation) of taxol was to induce almost complete inhibition of catecholamine release, whereas after a short incubation (10 rain) a massive, nicotine-independent release of catecholamine was produced. From results obtained using the patch-clamp technique to study the Ca+÷-dependent K ÷ channels (Ic channels), it was possible to conclude that taxol probably provokes an augmentation of free [Ca++]~ in the cytoplasm, values increasing from 10 -a M at rest to several 10 -7 M. The increased spontaneous release of stored neurohormones and the increased frequency of opening of Ic channels occur simultaneously and could both originate from a rise of [Ca++]~ upon taxol addition. Immunofluorescence and ultrastructural studies showed that 13-h taxol treatment of chromaffin cells led to a different distribution of secretory organelles, and also to microtubule reorganization. In treated cells, microtubules were found to form bundles beneath the cell membrane and, at the ultrastructural level, to be packed along the cell axis. It is concluded that in addition to its action on microtubules, the antitumor drug taxol has side effects on the cell secretory activity, one of them being to modify free [Ca++]i.