The proximate (protein, lipid, carbohydrate and ash) and elemental (C, H, N and P) composition of the major tissues were measured for 18 male and 51 female Illex argentinus sampled from the feeding grounds over the Patagonian Shelf. In most tissues the chemical composition did not vary with sexual maturity, although the mass of the tissue increased significantly because sexual maturation and growth were proceeding simultaneously. The composition of the ovary and associated tissues (nidamental gland, oviducal gland) did change significantly during sexual maturation. Several tissues contained significant amounts of one or more unknown components. The nitrogen content of an unknown component in the testis was similar to that of DNA. In the spermatophoric complex the nitrogen content suggested the unknown fraction may be an amino acid or short peptide, whereas in the nidamental gland the nitrogen content suggested an amino-sugar or polysaccharide derivative. The digestive gland was rich in lipid and continued to accumulate substantial reserves of energy throughout the period of sexual maturation on the feeding grounds. During this period there was no evidence for the utilization of either digestive gland or mantle tissues to supply energy for gonads. Accumulation of carbon and energy (estimated stoichiometrically from carbon) during the final 50 days on the feeding grounds indicated that energy demands for tissue synthesis in females were almost twice those of the smaller males, and that a relatively small fraction of the demands were for reproductive tissues (5% in males, 15% in females). Most energy intake in this period was directed to the digestive gland (40% in males, 47% in females) and other somatic growth (54% in males, 38% in females). A preliminary power budget suggested that during the final days of feeding before migrating to the spawning grounds, energy intake of Illex argentinus is 4-5% body energy content per day, growth efficiencies are low (17-22%) and that energy reserves in the digestive gland would fuel migration in the absence of feeding for 14 days in males and 21 days in females.