The effect of nifedipine (N) on sinus node (SN) function was studied in 15 patients (9 males, 6 females) sixty-two to seventy-six (mean 68.1 +/- 11) years old, with sick sinus syndrome (SSS). SSS was characterized electrophysiologically by a prolonged corrected sinus node recovery time (CSNRT greater than 535 msec) and/or prolonged sinoatrial conduction time (SACT greater than 125 msec), assessed by applying premature atrial stimulation. Ten mg N was given sublingually, and CSNRT and SACT were again evaluated sixty minutes after N administration, and again ten minutes after 1.5 mg atropine (A) was given IV. Heart rate increased significantly after N (p less than 0.005), systolic blood pressure (SBP) diminished significantly (p less than 0.005), and CSNRT and SACT shortened significantly (p less than 0.005, p less than 0.005) and became normal in 7 and 5 patients respectively. After A administration, a further significant increase of heart rate (p less than 0.005) and decrease of CSNRT (p less than 0.005) and SACT (p less than 0.005) were observed. CSNRT and SACT became normal in 8 and 7 patients respectively. SBP remained stable.