Abstract Qualitative and quantitative analyses of the fatty acid composition of fat body and haemolymph lipids have been conducted at two developmental stages in the life of Hyalophora cecropia. The main fatty acids present were C 16, C 16:1, C 18, C 18:1, C 18:2, and C 18:3, with traces of C 14, C 16:2, C 17, and C 20. During adult development there is a decline in the relative amount of polyunsaturated fatty acids in the fat body. The relative concentration of C 16 increases in the haemolymph lipids during pharate adult development, with a concomitant decrease in C 18:3. In vitro experiments revealed that most of the fatty acids released from the fat body into the haemolymph are in the diglyceride fraction, regardless of developmental stage. The use of double-labelled glycerides in release experiments demonstrated a difference in the composition of fat body glycerides, and of haemolymph glycerides derived from the fat body. The data suggest that neutral lipids released from the fat body into the haemolymph are restricted to those with a specific fatty acid composition, or that the fat body contains two different glyceride pools, only one of which communicates with the surrounding medium. A two-compartment theory is presented that favours the latter alternative.