Purpose: We test the reproducibility of human cardiac phosphorus magnetic resonance spectroscopy (31P-MRS) at ultra-high field strength (7T) for the first time. The primary motivation of this work was to assess the reproducibility of a ‘rapid’ 6½ minute 31P threedimensional chemical shift imaging (3D-CSI) sequence which, if sufficiently reproducible, would allow the study of stress-response processes. We compare this to an established 28 minute protocol, designed to record high quality spectra in a clinically-feasible scan time. Finally, we use this opportunity to compare the effect of per-subject B0 shimming on data quality and reproducibility in the 6 ½ min protocol. Methods: 10 healthy subjects were scanned on two occasions: one to test the 28 minute 3DCSI protocol, and one to test the 6½ minute protocol. Spectra were fitted using the OXSA Matlab toolbox. The phosphocreatine to adenosine triphosphate concentration ratio (“PCr/ATP”) from each scan was analysed for intra- and inter-subject variability. The impact of different strategies for voxel selection was assessed. Results: There were no significant differences between repeated measurements in the same subject. For the 28 min protocol, PCr/ATP in the midseptal voxel across all scans was 1.91 ± 0.36 (mean ± inter-subject SD). For the 6½ min protocol, PCr/ATP in the midseptal voxel was 1.76 ± 0.40. The coefficients of reproducibility (CR) were 0.49 (28 min) and 0.67 (6½ min). Per-subject B0-shimming improved the fitted PCr/ATP precision (for 6½ min scans), but had negligible effect on the coefficients of reproducibility (0.67 vs 0.66). Conclusions: Both 7T protocols show improved reproducibility compared to a previous 3T study by Tyler et al. Our results will enable informed power calculations and protocol selection for future clinical research studies.