We present estimates for head and body length (HBL) of fossil rodents. We assembled HBL data and tooth row length data (LTR, UTR) for a large number of extant rodents, and calculated regression lines of HBL on LTR and UTR for all rodents together (all-rodents equation), and for separate taxonomic groups (family equations). In fossil rodents, data for complete tooth rows are scarce, therefore we use the sum of the lengths of the teeth (LRsum, URsum) as a surrogate for tooth row length. The relation between this parameter and real tooth row length (LTR, UTR) is calculated on the basis of a number of populations for which both parameters are available. We estimate HBL of fossil rodents, using LRsum and URsum and the regression lines of extant rodents, and we compare the results for lower and upper tooth row when both are available. For each species we calculate HBL through the all-rodents equation and through the family equation. We consider the amount of difference between these two values as a measure of the reliability of this method.