Equilibrium [3H]ryanodine binding assay was applied to total membrane fractions of six rodent species, including the Mexican volcano mouse Neotomodon alstoni alstoni, Wistar rat Rattus norvegicus albinus, golden hamster Mesocritus auratus, gerbil Meriones unguiculatus, guinea-pig Cavia porcellus, and ground squirrel Spermophillus mexicanus. The organs selected for this study were: skeletal muscle, heart, brain and liver. The constants derived from Scatchard analysis show slight variations in their Kd, ranging from 3 to 15 nM, except in the gerbil's skeletal muscle (38 nM) and the hamster's brain (27 nM). Remarkably, the Bmax calculated in guinea-pig muscle was as high as that reported for the rabbit fast twitch muscle (4.6 pmol/mg of protein) using the same membrane fraction preparation. For all the other skeletal muscles, Bmax was similar to the corresponding heart Bmax values (from 0.5 to 1 pmol/mg of protein). Gerbil cardiac Bmax was the highest (1.1 pmol/mg of protein). The ground squirrel was the rodent with more cerebral ryanodine binding sites (0.26 pmol/mg of protein), whereas the rat and the volcano mouse showed the lowest values (0.12 pmol/mg of protein). The richest sources of hepatic ryanodine receptor were the guinea-pig and rat livers (approximately equal to 0.35 pmol/mg of protein), whereas the lowest Bmax corresponded to the hamster liver (0.018 pmol/mg of protein). These results allow us to detect the similarities and differences of the ryanodine receptor binding constants from four different tissues of some of the rodents most widely used as biomedical laboratory animals.