Effects of plane of nutrition, growth hormone treatment and dietary polyunsaturated fat on mammary development were assessed in prepubertal ewe lambs. Ten lambs (15.6 kg initial body weight) were assigned to each of four treatment groups. Treatments included: (A) lambs given ad libitum access to a high-energy ration; (G) fed as group A but injected s.c. daily with bovine growth hormone (.08 mg/kg BW); (R) feed intake restricted to achieve a growth rate of about 120 g/d; and (S) given ad libitum access to a ration including a formaldehyde-protected sunflower seed supplement. Diets were isonitrogenous and isocaloric and were fed from about 7 to 22 wk of age. At slaughter, the udder was removed and divided into halves. One udder half was trimmed and the parenchyma was dissected from surrounding stroma for compositional analyses. Final body weight and rate of gain averaged 41.2, 42.8, 27.3 and 46.0 kg, and 251, 267, 112 and 311 g/d for groups A, G, R and S, respectively. In the same treatment order, mammary parenchymal weight averaged 15.3, 20.3, 14.2 and 25.6 g. Parenchymal dry, fat-free tissue and DNA content were 1.7, 2.5, 1.3 g and 12.6, 24.1, 10.4 and 18.8 mg, respectively. Mammary parenchymal development was stimulated by polyunsaturated fat but was not affected by plane of nutrition. Supplemental dietary lipid may promote mammogenesis in ruminants.