The fatty acid content of fetal tissues was utilized to estimate essential fatty acid accretion during intrauterine growth. These rates of essential fatty acid accretion were used to predict that 400 mg/kg of body weight for omega-6 fatty acids and 50 mg/kg of body weight for omega-3 fatty acids per day would be used for intrauterine de novo synthesis of tissues. For the high risk low birth weight infant of approx. 1300 g birthweight it can be calculated that extrauterine de novo tissue synthesis would utilize about 280 mg/kg of body weight for omega-6 fatty acids and about 35 mg/kg of body weight for omega-3 fatty acids based on projected growth rates that are similar in composition to intrauterine growth and amount to 17 g of gain per day. From fatty acid analysis of brown and white adipose tissue it was estimated that some 70 and 78% of these net whole body accretion rates for essential fatty acids represent omega-6 and omega-3 fatty acid accretion in the adipose organs. Fatty acid analysis of human milk at day 16 of lactation was utilized to estimate the low birth weight infants' daily intake of major essential and other fatty acids. For intake levels of 120 kcal/kg it was concluded that the essential fatty acid content of mothers' own milk provides 6650 +/- 345 mg of total fatty acids, 850 +/- 160 mg of omega-6 fatty acids and 140 +/- 17 mg of omega-3 fatty acids.