We have developed a model of the tetrameric ryanodine receptor--the calcium channel of the sarcoplasmic reticulum. The model accurately describes published experimental data on channel activity at various concentrations of Ca2+, caffeine and quercetin. The proposed mechanisms involve allosteric regulation of Ca2+ affinity by both caffeine and quercetin, and the existence of two independent, A- and I-gates controlled by Ca2+ binding to an activating and an inhibitory module of the receptor. There are four different configurations of the receptor that affect ligand binding to the activation module, but not to the inhibition module. Consequently, there are four kinetic modes for the A-gate and one mode for the I-gate. At a certain moment, the receptor can be in any of the four possible conformations with equal probability. By fitting the data we are able to derive ligand affinities and Hill coefficients, to describe the observation that quercetin is an activating agent stronger than caffeine, and that caffeine and quercetin activate the channel at very low Ca2+ concentration (approximately 10(-11) M). We predict that the activation regime at saturating caffeine or quercetin should present four distinct regions at increasing Ca2+, corresponding to the four different gating modes. Another interesting prediction is the enlargement of the activity domain toward higher Ca2+ concentrations in the presence of caffeine or quercetin.