In the guinea-pig pancreas, 5-hydroxy-tryptamine (5-HT) occurs in the B-cells as well as in enterochromaffin cells scattered in the exocrine parenchyma. In the present study we examined the effects on the pancreatic 5-HT and insulin content of drugs known to affect the B-cells. For this purpose a radioimmunoassay of guinea-pig insulin was developed. Streptozotocin reduced the number of detectable B-cells and partially degranulated those that remained. It also lowered the insulin content of the pancreas and the 5-HT content of the B-cells but did not affect the 5-HT content of the enterochromaffin cells. Reserpine lowered the 5-HT content of both B-cells and enterochromaffin cells. The number and ultrastructure of the B-cells and the pancreatic insulin content was not affected. Streptozotocin + reserpine seemed to have additive effects of B-cell 5-HT. The results with these 2 drugs indicate that roughly 50% of pancreatic 5-HT is contained in the B-cells. Diazoxide markedly increased the pancreatic insulin content without affecting pancreatic 5-HT. Despite the fact that 5-HT and insulin are stored together in the cytoplasmic granules of the B-cells, 5-HT was differentially depleted from this store by reserpine. A marked disproportionality between 5-HT and insulin content was also induced by diazoxide.