OBJECTIVE To describe the allometric scaling principles underlying appropriate indexing of cardiovascular and respiratory measurements obtained in adult mammals, and to propose guidelines for indexing experimental cardiovascular and respiratory data. METHODS PubMed, using the terms 'allometry', 'allometric', 'indexing', 'cardiovascular' and 'respiratory'. CONCLUSIONS Indexing of cardiopulmonary variables is commonly used in attempts to account for the effects of body size on measurements and to standardize them. Some cardiopulmonary variables have been indexed using various functions of body mass in a process that often ignores the underlying relationship between the variable of interest and body size, as described in the allometry literature. This can result in a failure to ideally reduce the effect of body size on measurements in a manner that highlights differences. We review how commonly measured cardiopulmonary variables are related to body mass in mammalian species according to the allometry literature, and offer suggestions on how this information can be used to appropriately index cardiopulmonary variables in a simple and informative manner.