A suspension of tobacco cells,Nicotiana tabacum L. BY-2, was subjected to a rapid change of medium, resulting in disturbance of growth. A subpopulation of growing cells responded to such a nutritional signal by establishing a transient, localized Ca(2+) accumulation, as judged by chlorotetracycline fluorescence. Residing near or at the plasma membrane, this initial Ca(2+) signal began to relax after 1 h to a value presumably corresponding to an equilibrium Ca(2+) level. This response was susceptible to treatment with brefeldin A, an agent impacting vesicular traffic, as indicated by a further increase in fluorescence. By contrast, undisturbed growing and non-growing cells did not display a Ca(2+) response, regardless of the presence of brefeldin A.