Relative effects of equihypotensive doses (-35 mmHg) of adenosine (5.0 mumol/kg) and nitroglycerin (25 micrograms/kg) on heart rate and, therefore, baroreflex sensitivity were studied in conscious dogs. Nitroglycerin increased heart rate 133 +/- 24% from 78 +/- 5.5 beats/min, whereas adenosine increased heart rate only 79 +/- 16% from 78 +/- 5.2 beats/min (P less than 0.01). Injection of nitroglycerin during combined beta-adrenergic and muscarinic receptor blockades caused arterial pressure to fall 38 +/- 3.4% from 107 +/- 3.2 mmHg without any significant change in heart rate (3.8 +/- 3.8 from 162 +/- 9.2 beats/min). During combined beta-adrenergic and muscarinic receptor blockades adenosine also reduced arterial pressure 45 +/- 2.7% from 106 +/- 2.9 mmHg but unexpectedly reduced heart rate as well by 37 +/- 1.7% from 160 +/- 9.7 beats/min. This bradycardia reflected an effect on the sinoatrial (SA) node rather than an induction of heart block, since the R-R interval increased by 70 +/- 7.8% from 371 +/- 20 ms (P less than 0.01), while the P-R interval increased only 13 +/- 2.3% from 97 +/- 7.2 ms (P less than 0.05) with no electrocardiographic evidence of nonconducted beats. Arterial plasma adenosine levels were 43 +/- 5 nmol/ml at this time. Adenosine also caused bradycardia during muscarinic blockade alone (-43 +/- 3.4% from 201 +/- 6.4 beats/min) and following bilateral vagal section (-33 +/- 1.9% from 151 +/- 5.9 beats/min). In summary, adenosine appears to alter normal baroreflex function in the conscious dog by reducing the tachycardia that normally follows a fall in systemic arterial pressure.(ABSTRACT TRUNCATED AT 250 WORDS)