The body surface areas (BSAs) of 42 cotton rats (21 females weighing 29.08 to 169.60 g, and 21 males weighing 18.93 to 168.03 g) were measured. From these areas and body weights (BW), the equation, BSA (cm2) = 47.66 + 2.476 x BW (g) -0.006 x BW (g)2, was derived. To verify the applicability of the equation, the BSAs of another 19 animals (12 females weighing 29.08 to 169.60 g, and 7 males weighing 19.27 to 167.32 g) were calculated and the values compared with the measured BSAs. The correlation between calculated and measured BSAs was significant (r = 0.995, P < 0.01). Therefore, this equation can be used to calculate cotton rat BSA.