Myocardial architecture – the orientation and organisation of myocytes, fibrocytes and supporting collagen fibres in the three-dimensional myocardium – determines the spatial pattern of propagation and contraction within the heart. Cardiac diffusion tensor magnetic resonance imaging (DT-MRI) provides a non-destructive measure of the magnitude (eigenvalues λ1, λ2, λ3) and direction (their eigenvectors) of proton (water) diffusion in three dimensions. The eigenvectors have been used to quantify ventricular myofibre orientation and organisation (Gilbert et al., 2011). In an isotropic tissue λ1~λ2~λ3 and diffusion from a point source is into a sphere; in a cylindrically anisotropic tissue λ1>λ2~λ3 diffusion is into an ellipsoid; in an anisotropic and orthotropic tissue λ1>λ2>λ3 and diffusion is into a flattened ellipsoid. The orientation of the ellipsoid long axis is taken as the myofibre orientation, and the flattened ellipsoid may be interpreted as the orientation of cleavage planes between myolamellae. The diffusive spread of voltage leads to anisotropy and orthotropy in propagation velocity. 11 human foetal hearts, 14 to 20 weeks gestational age, with wet weight from 0.26 – 1.41g, were fixed (10% formalin) and imaged (MRI, FLASH, DT-MRI) in fomblin using a 9.4T Bruker Biospin MRI as in Benson et al. (2011), with a spatial resolution of 100 – 200µm, that is fine enough for computational modelling of tissue electrophysiology. λ1>λ2>λ3, their eigenvectors and derived quantities (angles derived from eigenvectors and fractional anisotropy (FA) derived from eigenvalues) were calculated, 0 ≤ FA ≤ 1. The ventricular walls and septum of all the 11 human foetal hearts were anisotropic and orthotropic (Fig. 1), with a ratio of eigenvalues λ1:λ2:λ3 of ~ 8:2:1, and with mean FA<0.3. The mean FA of adult rat myocardium is 0.43 (±0.001) and is associated with anisotropy and orthotropy in propagation velocities in the ratio of 4:2:1 (Hooks et al, 2007), that can be simulated by diffusion coefficients in the ratio of 16:4:1.