The incidence of cardiac dysrhythmias was investigated in 60 patients undergoing dental operations under halothane anaesthesia following either hyoscine, glycopyrrolate or placebo as a supplement to nalbuphine for premedication. Forty-five percent of the patients given 6 micrograms/kg hyoscine exhibited cardiac dysrhythmias compared to 25 percent of the group given 4 micrograms/kg glycopyrrolate and to 5 percent in the placebo group. The difference between the first two groups was not significant. But there was a significant difference between these two groups and the placebo group in which only 5 percent of the patients had cardiac dysrhythmias. There was neither a connection between the frequency of cardiac dysrhythmias and the demographic characteristics of the patients nor with their PaCO2. The author recommends to avoid premedication with anticholinergic combined with halothane anaesthesia with spontaneous respiration during dental operations.