The in vitro effect of metoclopramide on plasma cholinesterase (PCHE) activity was studied to investigate a mechanism for metoclopramide-induced prolongation of succinylcholine action. The mean PCHE of the control samples was 0.86 +/- 0.02 unit.ml-1. PCHE activity in the presence of metoclopramide, at concentrations of 0.05, 0.10, 0.50, 1.0, 2.5 and 5.0 micrograms.ml-1, was reduced to 0.78 +/- 0.02, 0.69 +/- 0.04, 0.50 +/- 0.03, 0.39 +/- 0.02, 0.24 +/- 0.01 and 0.15 +/- 0.01 unit.ml-1, respectively. Our data demonstrated that PCHE activity was significantly depressed by metoclopramide at all concentrations studied (p less than 0.001). Our data also show that the concentration of metoclopramide required to inhibit 50 per cent of PCHE activity (I50) was 0.8 micrograms.ml-1 (2.4 x 10(-6) M). We recommend caution when succinylcholine and or ester type local anaesthetics are administered to patients who are also receiving metoclopramide, especially in high doses.