1. The aim of the present work was to study the possible modulatory role of nitric oxide (NO) on the positive inotropic effect induced by the beta-adrenoceptor agonist isoprenaline in myocardial contractility, and whether this modulation is altered by hypertension. 2. The study was performed using right ventricular strips from the hearts of 6-month-old male Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). The contractile force of electrically-stimulated ventricular strips was measured by a force-displacement transducer. 3. Isoprenaline (from 10 nmol l(-1) to 10 micromol l(-1)) induced a concentration-dependent increase in cardiac contractility in strips from both rat strains. This positive inotropic effect to isoprenaline was reduced by the NO donor sodium nitroprusside (SNP, 0.1 mmol l(-1)) in muscles from WKY rats and slightly increased in those from SHR. The SNP-induced increase in strips from SHR was abolished by superoxide dismutase (100 U ml(-1)). 4. N(G)-nitro-arginine-methyl ester (L-NAME, 0.1 mmol l(-1)) and 1H-[1,2,4]oxadiazolo[4,3]-quinoxalin-1-one (ODQ, 10 micromol l(-1)), respective inhibitors of NO synthase and guanylate cyclase, increased the response to isoprenaline in muscles from WKY rats, whereas it was unaltered in strips from SHR. 5. In strips from WKY rats, the combination of ODQ and SNP produced an increase in the response elicited by isoprenaline, which was similar to that observed with ODQ or L-NAME. 8-Br-cyclicGMP (8-Br-cGMP, 0.1 mmol l(-1)), a permeable and structural cGMP analogue, decreased the effect induced by isoprenaline only in muscles from WKY rats. 6. These results suggest that the positive inotropic response to isoprenaline in ventricular strips from WKY rats is negatively modulated by NO, and positively by superoxide anions in those from SHR. The lack of a modulatory response to NO in ventricular strips from SHR is probably a result of an alteration of mechanisms in NO-signalling pathway downstream of cGMP formation in SHR hearts.