The EnSite™ intracavitary probe system developed by Endocardial Solutions, Inc (St. Jude Medical, St. Paul, MN) was used to simultaneously record geometric information, probe potentials, and selected endocardial potentials within the right atria for four patients. Tikhonov regularization was then used to estimate endocardial potentials from probe measurements for each patient at each endocardial site. The correlation coefficients and relative errors between the estimated potentials and the measured endocardial potentials were then calculated. This inverse problem was quite ill-conditioned, and first-order Tikhonov regularization performed better than zero-order or second-order Tikhonov regularization in producing stable and accurate results. In choosing the regularization parameter μ, a constant value of μ=0.3 performed as well as CRESO and Maximum Curvature, which pick a different μ for each time instant.