Pituitary adenylate cyclase activating polypeptide (PACAP) is a new member of the secretin/VIP peptide family. In order to identify pituitary target cells for PACAP, we examined the effect of PACAP on the cytosolic-free Ca2+ concentrations, [Ca2+]i, in cells using Fura-2 and a digital imaging system. In a concentration-dependent manner, PACAP increased [Ca2+]i in pituitary cells, from 10(-11) to 10(-8) M. Some cells responded to PACAP at 10(-11) M, while others only at 10(-8) M, showing a diversity among cells in the sensitivity to PACAP. Both PACAP and growth hormone-releasing factor (GRF) at 10(-8) M increased [Ca2+]i in 25% and 33% of the total pituitary cells, respectively. About 38% of the GRF-responsive cells also responded to PACAP. Some of the cells that responded to PACAP were immunocytochemically identified as growth hormone (GH) cells or folliculo-stellate (FS) cells using antisera against GH or S-100 protein, respectively. The results indicate that both the folliculo-stellate cells and GH cells of the pituitary are the direct targets for PACAP, but the possibility that other cell types also respond to PACAP cannot be excluded.