1. The functional consequences of a lack of endothelial nitric oxide synthase (eNOS) on left ventricular force development and the anti-adrenergic effect of acetylcholine (ACh) were investigated in isolated hearts and cardiomyocytes from wild type (WT) and eNOS knockout (eNOS-/-) mice. 2.eNOS expression in cardiac myocytes accounted for 20 % of total cardiac eNOS (Western blot analysis). These results were confirmed by RT-PCR analysis. 3. In the unstimulated perfused heart, the left ventricular pressure (LVP) and maximal rate of left ventricular force development (dP/dtmax) of eNOS-/- hearts were not significantly different from those of WT hearts (LVP: 97 +/- 11 mmHg WT vs. 111 +/- 11 mmHg eNOS-/-; dP/dtmax: 3700 +/- 712 mmHg s(-1) WT vs. 4493 +/- 320 mmHg s)-1) eNOS-/-). 4. The dobutamine (10-300 nM)-induced increase in LVP was enhanced in eNOS-/- hearts. In contrast, L-type Ca2+ currents (ICa,L) in isolated cardiomyocytes of WT and eNOS-/- hearts showed no differences after beta-adrenergic stimulation. Dibutyryl-cGMP (50 microM) reduced basal ICa,L in WT cells to 72 +/- 12 % while eNOS-/- ICa,L was insensitive to the drug. The pre-stimulated ICa,L (30 nM isoproterenol) was attenuated by dibutyryl-cGMP in WT and eNOS-/- cells to the same extent. 5. The Ca2+ (1.5-4.5 mM)-induced increase in inotropy was not different between the two experimental groups and beta-adrenergic receptor density was increased by 50% in eNOS-/- hearts. 6. The contractile effects of dobutamine could be inhibited almost completely by ACh or adenosine. The extent of the anti-adrenergic effect of both compounds was identical in WT and eNOS-/- hearts. Measurement of ICa,L in isolated cardiac myocytes yielded similar results. 7. These data demonstrate that in the adult mouse (1) lack of eNOS is associated with increased cardiac contractile force in response to beta-adrenergic stimulation and with elevated -adrenergic receptor density, (2) the unaltered response of ICa,L in eNOS-/- cardiac myocytes to beta-adrenergic stimulation suggests that endothelium-derived NO is important in mediating the whole-organ effects and (3) eNOS is unimportant for the anti-adrenergic effect of ACh and adenosine.