Protein-bound uremic toxins such as indoxyl sulphate (IXS) accumulate in the blood of patients with chronic kidney disease (CKD) and are poorly cleared by dialysis. IXS is associated with cardiovascular mortality and incidence of heart failure in CKD. Its mechanisms of action are unclear but it has been reported to increase reactive oxygen species (ROS) in endothelial cells (Dou et al., 2007). In this study we tested whether IXS generates ROS and modulates contractility in ventricular myocytes. Single ventricular myocytes were isolated from the hearts of male Wistar rats. Cells were loaded with the ROS indicator CM-H2DCFDA (DCF) for 20 minutes at a concentration of 2 µM. Cells were superfused with HEPES-buffered Tyrode’s solution then exposed to this solution in the presence or absence of IXS (200 µM) or H2O2 (50µM) for 2 minutes at 37 ± 1oC. Cells were briefly exposed to excitation light of 475 nm (Optoscope, Cairn Research) and emitted fluorescence was measured at 510 nm. DCF fluorescence and sarcomere length were simultaneously measured in myocytes that were stimulated to contract by external Pt electrodes at a frequency of 1Hz. Sarcomere length was measured by fast Fourier transform of a video image of the cell (IonWizard, IonOptix). After 2 minutes exposure, DCF fluorescence in response to Tyrode fell to 0.91 ± 0.04 of initial fluorescence, (n = 20 myocytes), in response to IXS, mean fluorescence was 1.00 ± 0.03 (n = 35 myocytes) and after exposure to H2O2 fluorescence was increased to 1.08 ± 0.05 (n = 29 myocytes), (P < 0.05 Tyrode vs H2O2, 1-way analysis of variance). However in response to IXS, there was a statistically significant, positive correlation between the change in fluorescence and change in sarcomere length shortening (Pearson’s correlation coefficient 0.53, P < 0.05). In 22 of 31 myocytes exposed to IXS, fluorescence was above the mean value for Tyrode exposure (0.91). In these myocytes sarcomere shortening increased by 16 ± 6 %. In contrast, in 9 myocytes exposed to IXS where fluorescence was less than 0.91, sarcomere shortening fell by 19 ± 6 % (P = 0.002, unpaired t-test). We conclude that when exposure to IXS leads to generation of ROS in ventricular myocytes there is an associated increase in contraction. This may be linked to a c’AMP-independent activation of protein kinase A (Burgoyne et al., 2012). Chronic stimulation of ROS production or PKA activation by IXS could be detrimental to the myocardium in CKD. Further study to understand the variable response of myocytes to IXS is in progress.