Intraperitoneal fat body (IPF) was biochemically and morphologically compared between wild and cultured ayu in relation to lipid mobilization. In vitro lipolysis activity was preliminarily determined using IPF obtained from cultured ayu. IPF preparations (homogenates, fat cells, and slices) were incubated with bovine serum albumin with lipolytic hormones. Lipolytic hormones induced marked fatty acid release from IPF. Although cyclic AMP (c-AMP) did not stimulate the activity, dibutyryl-AMP induced fatty acid liberation. These results suggest the presence of a hormone-sensitive lipase and a possible role of c-AMP in phosphorylation by protein-kinase in lipolysis. In the in vitro lipolytic system, incubation with IPF homogenates from both wild and cultured ayu liberated preferentially C22:6. However, incubation with IPF slices liberated all fatty acids non-selectively. In vitro lipolysis activity of cultured ayu was markedly lower than that of wild ayu. Scanning electronic microscopic observation of IPF of cultured ayu showed significantly larger cell diameter than in that of wild ayu.