Atrial pacing at progressively increasing frequencies was performed in 12 awake dogs through electrodes implanted aseptically in the right atrium and led out through the skin of the neck. Heart rate and the Wenckebach point (the minimum pacing frequency for which a second degree A-V block first appeared) were measured by electrocardiography. The responses of the sino-atrial and auriculo-ventricular nodes to atropine and hyoscine methobromide were characteristically bimodal. Slowing of the heart rate and atrioventricular conduction appeared with the lower doses of atropine (6.25 micrograms kg-1 i.v) and hyoscine methobromide (0.4 micrograms kg-1 i.v.); at doses of atropine 12.5 micrograms kg-1 i.v. and hyoscine methobromide 1.56 micrograms kg-1 i.v., only atrioventricular conduction was slowed; an acceleration of heart rate and atrioventricular conduction occurred in response to the higher doses of both drugs. These results show, in the awake dog, that the effects of atropine and hyoscine methobromide on heart rate and atrioventricular conduction are entirely comparable when appropriate doses are used, and reveal a difference in the responsiveness of the sino-atrial and auriculo-ventricular nodes to these drugs.