Glucose-induced insulin release is markedly decreased in the spontaneously diabetic Goto-Kakizaki (GK) rat pancreas. This defect was recently shown to be reversed by forskolin which markedly enhances cAMP generation in GK islets. These effects of forskolin were associated with overexpression of type-3 adenylyl cyclase (AC) mRNA due to the presence of two functional point mutations in the promoter region of AC3 gene in GK rat. Nine AC isoforms have been described, but their expression pattern in relation to the main pancreatic islet cell types, as well as their involvement in the diabetic state, is still unknown. Using antibodies raised against AC1-8, we have studied by double immunofluorescence the localisation of these AC isoforms in different endocrine cell types in both normal and diabetic GK rat pancreas. Our results demonstrated a clear immunoreaction (IR) to AC1-4 and 6 in normal and GK islet beta-cells, while a smaller number of ACs were expressed in alpha- and delta-cells. No AC-IR was observed in pancreatic polypeptide cells. Moreover, we have found an increased IR of the Ca2+-stimulated ACl, AC3 and AC8 in diabetic beta- and alpha-cells, compared with the corresponding IR in control pancreas. Most noticeable was the eliciting of a markedly enhanced AC8-IR in GK rat beta- and alpha-cells, in contrast to a barely discernible AC8-IR in corresponding normal cells. In conclusion, AC expression exhibits a complex pattern in the endocrine pancreas, with specific differences between the normal and diabetic state.