WITH regard to A. E. Needham's communication1 stressing the desirability of obtaining relative growth data on individuals as distinct from contemporaneous data obtained in populations, we would like to point out that such data relating to guinea pigs were published by us some time ago2. We then demonstrated the isometry of teat–length against body–length in the young male guinea pig and the occurrence of ‘simple’ allometry of the teat under androgenic stimulation. In these experiments data for groups of comparable individuals were averaged.