Current voltage (I-V) relations were measured from the calcium release channel (CRC) of the sarcoplasmic reticulum of cardiac muscle in 12 KCl solutions, symmetrical and asymmetrical, from 25 mM to 2 M. I-V curves are nearly linear, in the voltage range +/- 150 mV approximately 12kT/e, even in asymmetrical solutions, e.g., 2 M // 100 mM. It is awkward to describe straight lines as sums of exponentials in a wide range of solutions and potentials, and so traditional barrier models have difficulty fitting this data. Diffusion theories with constant fields predict curvilinear I-V relations, and so they are also unsatisfactory. The Poisson and Nernst-Planck equations (PNP) form a diffusion theory with variable fields. They fit the data by using adjustable parameters for the diffusion constant of each ion and for the effective density of fixed (i.e., permanent) charge P(x) along the channel's "filter" (7-A diameter, 10 A long). If P(x) is described by just one parameter, independent of x (i.e., P(x) = P0 = -4.2 M), the fits are satisfactory (RMS error/RMS current = 6.4/67), and the estimates of diffusion coefficients are reasonable D(K) = 1.3 x 10(-6) cm2/s, D(Cl) = 3.9 x 10(-6) cm2/s. The CRC seems to have a small selectivity filter with a very high density of permanent charge. This may be a design principle of channels specialized for large flux. The Appendix derives barrier models, and their prefactor, from diffusion theories (with variable fields) and argues that barrier models are poor descriptions of CRCs in particular and open channels in general.