When cardiac muscle is subjected to stretch the force of contraction increases, allowing the intact heart to adjust its output to the body’s demand (Allen & Kentish, 1985). This increase in contractility has been shown in vivo to occur in two distinct phases. Initially there is an abrupt increase in force that coincides with the stretch, and secondly there is a slower response that develops over a period of a few minutes (the ‘slow force response’). The first of these responses is largely due to a change in the sensitivity of the contractile proteins to Ca²⁺, whereas the slow force response is accompanied by a concomitant increase in the magnitude of the intracellular Ca²⁺ transient (the event that initiates contraction). It has been proposed that stretch-activated channels contribute to Ca²⁺ entry after stretch (Calaghan & White, 2004). The aim of the present study was to reinvestigate the mechanisms underlying the slow force response of cardiac muscle. Mice (C57 BL10) were killed by intraperitoneal injection of 100 mg/kg body wt sodium pentobarbitone, mixed with 10 mg/kg body wt heparin to prevent blood coagulation, and hearts rapidly removed. Cardiac trabeculae, or small diameter papillary muscles (< 1 mm in length, and 0.1-0.3 mm in diameter), were dissected from the right ventricle, and mounted in a muscle chamber between a hook attached to a force transducer and a lever connected to a motor capable of making precise changes in muscle length. Each preparation was then subjected to a step increase in length for 2 min whilst isometric force was recorded. One minute after the initial length change active force increased by 77 ± 17% of the force immediately following the stretch (n = 16). Subsequent application of either 400 µM streptomycin, or 20 μM GdCl₃ (blockers of stretch-activated channels) reduced the slow force response (p ≤ 0.01, paired t-test) for identical step increases in length (streptomycin: from 86 ± 25% to 38 ± 14% (n=9), or GdCl₃: from 65 ± 21% to 12 ± 7%, n=7), suggesting a possible role for stretch-activated channels in the slow force response. The involvement of these channels was further tested by incubating preparations with 10 μM GsMTx-4, a specific blocker of stretch-activated channels (Suchyna et al. 2000). In the presence of GsMTx-4 the slow force response 1 min following the stretch was reduced from 112 ± 40% in the absence of the toxin, to 15 ± 8% (n=6, p ≤ 0.01 Mann-Whitney Rank Sum test). These results confirm a role for stretch-activated channels in the slow force response of mouse myocardium.