Tyrosine hydroxylase (TH) is co-expressed with islet hormones in the fetal mouse pancreas. In the adult animal, the enzyme has been considered as a marker of ageing beta-cells. By immunohistochemical staining, we analyzed the expression of TH-like immunoreactivity (TH-LI), insulin-LI (INS-LI) and somatostatin-LI (SOM-LI) in adult mouse islets, in situ and after isolation and transplantation to kidney. In pancreas in situ, most TH-LI cells expressed INS-LI while less than 5% expressed SOM-LI. The total number of TH-LI cells/mm2 was significantly increased directly after isolation and in 0-day, 12-week and 52-week old grafts, but not in 3-day grafts. The proportion of TH-LI cells expressing SOM-LI increased after transplantation, amounting to about one-third by 52 weeks. As expressed per unit islet area, the frequencies of both TH/INS and TH/SOM cells increased significantly in the transplants. The results demonstrate that TH occurs in both beta-cells and D-cells of adult islets. In both cell types the enzyme appears to be responsive to the microenvironmental changes inherent in transplantation. This cellular phenotype plasticity might contribute to the altered insulin secretory dynamics in islet grafts.