1. Newborn rats were reared in litters of either four or sixteen individuals. The animals from the small litters gained body weight more rapidly than those from large litters during the first 29 days of postnatal life studied. 2. The relative weights of the perigenital, perirenal, subcutaneous and intramuscular white-adipose-tissue sites in the animals from small litters indicated their relative obesity compared with controls. 3. The adipose depots from animals reared in small litters had a greater proportion of lipid present, by weight, and had a greater number of larger fat-cells present in them compared with the depots of animals reared in large litters. 4. Compared with both normal-sized litter controls and animals reared in sixteens, during the period of study the animals from small litters were hypertriacylglycerolaemic but normocholesterolaemic. 5. During suckling the blood glucose concentrations of animals reared in fours were increased, as were the concentrations of circulating immunoreactive insulin. 6. During the 29 days of life studied, in general, the lipoprotein lipase activity of adipose depots from animals reared in fours was greater than for animals in large litters when expressed as μmol of nonesterified fatty acid released from the substrate/h per g fresh weight of tissue, per depot, or per million fat-cells, but were similar per cm2 of fat-cell surface area. 7. The previously noted [Cryer & Jones (1978) Biochem. J.172, 319–325] pattern of mid-suckling elevation, late-suckling decline and post-weaning increase in the lipoprotein lipase activity of the four white-adipose depots studied was not obliterated by the nutritional manipulations employed. 8. The relation of the enzyme-activity changes and their hormonal stimuli to triacylglycerol accumulation in fat-cells of animals from large and small litters is discussed in relation to the possible significance they may have to our understanding of neonatally induced obesity.

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