An investigation was made of the effect of histamine on amylase secretion and calcium mobilization in isolated guinea-pig pancreatic segments. The effect of acetylcholine (ACh) was also examined for comparison. Histamine evoked a dose-dependent increase in amylase output from superfused segments. It had small transient effects on both 45Ca influx and efflux, and elevated cytosolic free Ca2+ concentration in acini. The equivalent dose of ACh evoked a greater amylase output and greater changes in Ca2+ mobilization. The results suggest that histamine may have a physiological role in exocrine secretion of the guinea-pig pancreas but is less potent than ACh.