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Introduction: Information on the force-length relation of intact myocytes from different regions of the heart is scarce. We therefore studied myocytes, isolated from apical and basal areas of guinea pig left and right ventricles (cell numbers: LVA 22, LVB 29, RVA 11, RVB 12). Methods: Force-length relations were measured by attaching carbon fibers to myocytes, allowing application of diastolic stretch while measuring passive and active force.1 Cells were kept at 36±1°C and paced at 2 Hz. Recorded forces were normalized to cell cross-sectional area and used to construct end-diastolic, end-systolic and active tension-length relations (EDTL, ESTL and ATL=ESTL-EDTL; respectively). In addition, the ratio of the slopes of ESTL and EDTL was used as a cross-section independent factor to characterise the Frank-Starling Gain (FSG) in individual cells. Results: For all tissue regions ESTL, EDTL and, hence, ATL, are linear over the range of end-diastolic sarcomere lengths studied (1.88-2.15 μm). Plotting the slope values of ATL vs. EDTL for all cells shows a positive correlation (slope 1.29, R2=0.24, 74 cells). In addition, FSG is larger than one for all cells studied: RVB 3.91±0.54), RVA (2.84±0.30), LVB (2.77±0.17), and LVA (3.20±0.24). Conclusions: Using the carbon fiber technique, it is possible to probe length-dependence of passive and active tension at the single cell level, without the interference of extracellular structures. The Frank-Starling Gain varies between the different regions of the heart and the positive correlation between ATL and EDTL confirms that in intact cells passive force bearing structures (such as titin) are likely to play a role in modulating length dependent activation.[1] Iribe et al, Force-length relations in isolated intact cardiomyocytes subjected to dynamic changes in mechanical load. AJP 2007/292:1487-1497.