The body surface area (BSA) of 50 adult 2/N guinea pigs of both sexes and varying age was measured by the skinning and paper weight technique, or calculated from two equations in which the animals' weight (W) alone or the weight and length (L) were known. The equations BSA = W2/3 x 8.054 and BSA = W0.425 x L0.725 X 3.545 were developed by the method of least squares. The differences between the values obtained by measurement and those obtained by either of the calculation methods were not statistically significant. Differences because of sex and age were also not significant.