Changes in the cytosolic free calcium ion concentrations ([Ca2+]i) induced by LHRH were studied in individual rat granulosa cells using fura-2 microspectrofluorimetry. The resting [Ca2+]i concentration was 96.7 +/- 2.9 nM (n = 115). The majority of the granulosa cells responded to LHRH at doses between 10(-7) and 10(-5) M; the latter dose was the highest concentration required to initiate alterations in [Ca2+]i in these cells. The alterations in [Ca2+]i induced by LHRH were transient and returned to resting levels within 84 +/- 3 sec (n = 64). A potent LHRH antagonist completely blocked the effect of LHRH on [Ca2+]i. Within a single granulosa cell, three consecutive injections of the same dose of LHRH, delivered 5 min apart, induced three discrete peaks of [Ca2+]i of similar amplitudes. Sustained perfusions of LHRH, however, resulted in a desensitization of the [Ca2+]i response to LHRH, but not to the calcium ionophore Br-A23187. These results, which were obtained from individual cells, provide strong support for the hypothesis that acute changes in [Ca2+]i are involved in the early cellular transduction of the LHRH signal in the ovary.