The authors described the effect of premature weaning on the fatty acid spectrum in the retroperintoneal fat of female rats during ontogenesis. Premature weaning reduced the lower fatty acid content [decanoic acid C 10:0, lauric acid C 12:0, myristic acid C 14:0] in 30- to 90-day-old rats, but in further development no differences between the amount of these fatty acids were observed in suckled and prematurely weaned animals. These changes concurred with the fatty acid composition of the relevant diet (rat milk and a standard laboratory diet). Changes induced in the amount of other acids by premature weaning were not permanent in character, with the exception of stearic acid C 18:0 and linoleic acid C 18:3, which were raised at 720 days in the retroperitoneal fat of prematurely weaned rats. The amount of lower saturated fatty acids fell with advancing age, whereas oleic C 18:1, linolic C 18:2 and linolenic C 18:3 acid rose.