Forty-one Holstein and 6 Holstein crossbred heifers, 6 to 8 mo of age, were used to determine the effect of plane of nutrition on growth and mammogenesis prior to and during puberty. Animals were fed to gain 611 g (low), 737 g (medium), and 903 g (high) by a diet of cracked corn and chopped alfalfa-grass hay. Mammary biopsies were carried out in vivo to determine if they provide acceptable information on mammary composition (based upon morphometric evaluation) in comparison with dissected glands. Results indicated that NRC (1978) recommendations for average daily gain of Holstein heifers between 6 and 16 mo may be too generous. At puberty (first estrus), age and wither height decreased linearly with increasing plane of nutrition, whereas body weight and hip height were not affected by plane of nutrition. Five heifers were slaughtered at the beginning and 18 at the conclusion of the trial. Increasing plane of nutrition resulted in fatter mammary glands with decreased concentration of DNA, whereas total mammary DNA did not differ among treatment groups. In this study, morphometric evaluation of mammary tissue obtained through biopsies did not yield useful information in comparison to chemical analysis of dissected glands.