When cardiac muscle is stretched a biphasic force response is observed: an immediate increase in force development, followed by a further slow increase in force (the slow force response, SFR). The mechanisms involved in the SFR are controversial. There is evidence that muscle stretch activates the Na-H exchange, leading to a rise in intracellular [Na⁺]_i and, via Na-Ca exchange, a rise in intracellular [Ca²⁺]_i (Alvarez et al., 1999). However, other results (Petroff et al., 2001) suggest that the SFR is caused by a rise in sarcoplasmic reticulum (SR) Ca release due to enhanced NO production in the stretched cardiac myocytes. We have therefore studied the effect of NOS inhibition on the SFR in isolated cardiac muscle. Rats were injected with sodium pentobarbitone (50 mg.kg^-1) and the hearts were rapidly excised. Trabeculae, isolated from the right ventricle, were bathed in Krebs solution at 23 °C and were stimulated electrically. The trabeculae were progressively stretched to a muscle length where twitch force was 95% of maximum (L₉₅). Following a stabilisation period, the trabeculae were shortened to a length giving 70% of maximum force (L₇₀) for 10 min, then were re-stretched from L₇₀ to L₉₅.The magnitude of the resulting SFR was assessed as the % increase in twitch force at 10 min following re-stretch compared with the force in the first minute after stretch. After a minimum of 3 control SFRs, the trabeculae were incubated in the NOS inhibitor, L-NAME (1 mM), for 20 min. The release/stretch protocol was then repeated and the SFRs recorded. The presence of L-NAME reduced the SFR from 137.0 ± 6.2 % to 123.8 ± 3.7 % (mean ± SEM, n=7, p<0.05, paired t-test). Conversely, application of the NO substrate, L-arginine (1 mM), to trabeculae increased the SFR from 139.3 ± 5.3 % to 156.4 ± 8.4 % (n=8, p0.5). To examine the role of the SR in the NO response, the SR was inhibited using ryanodine (1 µM) plus CPA (30 µM). During SR inhibition, the SFR was still present, but L-NAME did not reduce the magnitude of the SFR (n=4, p>0.5). These results suggest that release of NO is partly responsible for the SFR in rat cardiac muscle. This effect appears to be cGMP-independent but requires a functional SR.