Rats were raised on a low-fat diet containing 6 g fat/kg. Females of the second generation were bred and only 11 % of their pups survived to weaning age compared with a 66% survival for control pups. Pups were killed 8-12 h after birth and their tissues were analysed. Pups in the low-fat group had smaller body, brain and liver weights than control pups; the lipid contents of body, brain and liver were also significantly less. In the liver triglycerides from the control group the C20 and C22 polyenoic fatty acids constituted 33 % of the total fatty acids. The liver triglyceride concentration in the low-fat group was lower and the concentration of the long-chain polyenoic fatty acids in this fraction was 20 % of the control value. The milk fatty acids from the low-fat group contained only 33 % as much of the C18 to C22 polyenoic fatty acids compared with the control group. In the brain lipids from the low–fat group, changes in the fatty acid composition were less marked than in the liver lipids. In these experiments there were only small amounts of 20:3ω9 in the tissue lipids; the ratio to 20:4w6 was less than I. These changes are discussed in relation to the influence of dietary lipids on tissue growth especially of lipid-rich tissues such as the brain.