The ratio of short to long loop nephrons (SLNs and LLNs, respectively) in laboratory rodents (mice, rats, hamsters, gerbils, and guinea pigs) was investigated using the air cast method. In mice and rats, the percentage of SLNs was significantly higher than that of LLNs, while in hamsters and gerbils, the reverse was true (% of LLNs >% of SLNs). In guinea pigs, no significant difference in the percentages of LLNs and SLNs was noted.