The effects of truncal vagotomy on gastrin cell and somatostatin cell populations and circulating hormone levels were studied in rats. Truncal vagotomy produced significant gastrin cell hyperplasia, with the largest increase in gastrin cell density observed 2 weeks postvagotomy. Gastrin cell hyperplasia was sustained up to 6 months. Both antral somatostatin cell and corpus somatostatin cell numbers demonstrated significant and sustained increases after vagotomy; the largest increases were observed 2 weeks after operation. Early elevation of circulating gastrin to 3 times the baseline level was followed by sustained, lower level hypergastrinemia. Plasma somatostatin levels were decreased 2 days after vagotomy, with a return to control values 2 weeks after operation. Gastrin cells and somatostatin cells are dynamic populations, capable of changes in both number and secretory function after truncal vagotomy.