Receptor/ligand binding on a cell surface may activate the calcium signal transduction cascade, resulting in the release of calcium from intracellular stores into the cytosol. Changes in intracellular free calcium, [Ca2+](i), following ligand stimulation have been linked to a variety of cell responses,from muscle contraction to hormone secretion. We have monitored changes in [Ca2+](i) in Single smooth muscle-like BC(3)H1 cells following stimulation by the vasoconstrictor phenylephrine, using the fluorescent calcium probe, fura-2, in a digital fluorescence imaging system. We find that not all cells respond to ligand stimulation with changes in [Ca2+](i). In addition, cells which respond to ligand stimulation exhibit considerable heterogeneity in the speed of calcium mobilization for a given ligand concentration. Both the population-averaged speed for calcium mobilization and the fraction of cells which respond to ligand stimulation are increasing functions of the ligand concentration. In contrast, the magnitude of the ligand-stimulated increase in [Ca2+](i) from basal to peak levels in responding cells is independent of ligand concentration. We postulate that the heterogeneity seen in the ligand-induced mobilization of calcium among single cells is a function of distinct differences between cells, such as number of receptors, size of the intracellular calcium store, or phospholipase C activity. We have developed a mathematical model, based on the calcium signal transduction cascade, to predict single-cell calcium responses to ligand stimulation. We have systematically incorporated cell-to-cell parameter heterogeneity into the model by randomly selecting single-cell parameter values from a Gaussian distribution. Model simulations predict both single-cell and population-averaged trends that we have observed experimentally. The results of this work suggest that increases in a population response may be the result of increased participation in the response as opposed to increases in the magnitudes of individual cell responses.