We have studied the effects of extracellular nucleotides on the cytosolic free calcium concentration [( Ca2+]i) in J774 macrophages using quin2 and indo-1 as indicator dyes. Micromolar quantities of ATP induced a biphasic increase in [Ca2+]i: a rapid and transient increase (peak I) which was due to mobilization of Ca2+ from intracellular stores and a second more sustained elevation (peak II) due to influx of extracellular Ca2+. The sustained peak II elevation had two components, a "low threshold" (1 microM ATP) response which saturated at 10-50 microM ATP and a "high threshold" response, apparent at [ATP] greater than 100 microM. The latter component was not seen with nucleotides other than ATP and correlated with an ATP-induced generalized increase in plasma membrane permeability. A variant J774 cell line was isolated which does not demonstrate this ATP-induced increase in plasma membrane permeability; nevertheless, it demonstrated both the release of Ca2+ from intracellular stores and the low threshold component of the Ca2+ influx across the plasma membrane in response to nucleoside di- and triphosphates. Several lines of evidence indicate that the fully ionized (i.e. free acid) forms of nucleoside di- and triphosphates were the ligands that mediated these increases in [Ca2+]i. These data show that extracellular nucleotides mediate Ca2+ fluxes by two distinct mechanisms in J774 cells. In one, the rise in [Ca2+]i is due to release of Ca2+ from intracellular stores and Ca2+ influx across the plasma membrane. This response is elicited preferentially by the free acid forms of purine and pyrimidine nucleoside di- and triphosphates. In the other, the rise in [Ca2+]i reflects a more generalized increase in plasma membrane permeability and is elicited by ATP4- only.