Cardiac parasympathetic control has prognostic significance in heart failure, but the control mechanisms of this system remain poorly defined. We have demonstrated previously a facilitatory role for nitric oxide (NO) in the parasympathetic control of heart rate in young healthy human subjects. In view of the complex abnormalities of regional NO activity observed in chronic heart failure, we now aim to establish if this mechanism is active in subjects with this condition. Groups of 12 heart failure patients [NYHA class II-III; mean age 52 years (range 38-67 years)] and 12 age/sex-matched healthy control subjects [mean age 50 years (range 36-62 years)] were studied. Heart rate variability and baroreflex sensitivity were measured during inhibition of endogenous NO production with N(G)-monomethyl-l-arginine (l-NMMA; 3 mg.h(-1).kg(-1)) and during administration of an equipressor dose of the control vasoconstrictor phenylephrine (12-36 microg.h(-1).kg(-1)). Basal levels of nitrate+nitrite were measured in the plasma as an indication of systemic NO production. In the heart failure patients, despite an equal rise in blood pressure with both drugs, high-frequency indices of heart rate variability increased less with l-NMMA than with phenylephrine: RMSSD (root mean square of successive RR-interval differences) increased by 4+/-2 compared with 26+/-8 ms (P<0.001) and high-frequency power increased by 97+/-62 compared with 1372+/-861 ms(2) (P<0.001). The increases in cross-spectral baroreflex sensitivity were also lower with l-NMMA than with phenylephrine [high-frequency alpha-index, 2.2+/-1.3 and 12.6+/-3.8 ms/mmHg respectively (P<0.001); low-frequency alpha-index, 1.3+/-0.9 and 4.3+/-1.7 ms/mmHg respectively (P<0.05)]. Healthy subjects showed a similar discrepancy in the response of high-frequency indices of heart rate variability to the two drugs, although baroreflex sensitivity responses were significantly different only for the high-frequency alpha-index. Levels of plasma nitrate+nitrite were significantly higher in the heart failure patients compared with controls. These data demonstrate that baroreflex-mediated cardiac parasympathetic activation in human heart failure, as in health, is dependent upon endogenous NO synthesis.