Nitric oxide (NO) plays a modulatory role on cell growth and differentiation, biological processes that occur under the control of various signal transduction mechanisms, including those triggered by activation of membrane receptors for polypeptide growth factors. The increases in intracellular Ca2+ concentration elicited by the activation of these receptors are sustained by release of the cation from intracellular stores and by stimulation of this influx from the extracellular medium. Using NIH 3T3 cells overexpressing the human epidermal growth factor receptor, we investigated both of these processes stimulated by the administration of epidermal and platelet-derived growth factors as the receptor agonists. Pharmacological and functional analyses carried out on Fura-2-loaded cells showed that Ca2+ influx elicited by both growth factors is the summation of two distinct pathways, with the major pathway dependent on and the minor pathway independent of store depletion. Exposure of the cells to either No donors or NO synthase inhibitors induced increase and inhibition, respectively, of the two components of Ca2+ influx. When Ca2+ release was investigated, the above drugs were also active but in the opposite direction. The effects of NO were mimicked by the cGMP analogue 8-Br-cGMP and abolished by two cGMP-dependent protein kinase I inhibitors, whereas the cAMP analogue 8-Br-cAMP and two protein kinase A inhibitors had no appreciable effects. In addition, growth factors induced an increase in cGMP formation, an effect that was prevented by NO synthase inhibitors. In conclusion, NO appears to exert a feedback modulatory control on CA2+ responses to growth factor administration. Such a control might contribute to the inhibitory effect of NO on growth previously reported with various cell types.