In previous papers we have shown that high temperature and low food levels result in a decline in the body condition of mussels, Mytilus edulis, which have been kept in the laboratory (Bayne & Thompson, 1970; Gabbott & Bayne, 1973). In spite of the loss of body reserves, M. edulis is able to continue gonad development at an increased rate, when the temperature is above ambient (Gabbott & Bayne, 1973; Bayne, 1975). Although gametogenesis appears to be normal under these conditions, there is evidence that stress in the adult affects subsequent larval development in M. edulis (Bayne, 1972) and in the oyster, Ostrea edulis (Helm, Holland & Stephenson, 1973). In mussels, stress resulted in an increase in abnormal embryonic development during cleavage, gastrulation and development to the first shelled larval stage (Bayne, 1972). In oysters the viability of the larvae, assessed in terms of growth rate and percentage yield of spat, was less in larvae from adults kept at low ration than in larvae from adults kept at high ration. Irrespective of the feeding regime, the viability of the larvae fell as the length of the conditioning period increased, and this was related to a decline in the body condition of the adult oysters. The growth of the larvae in the 96 h period following liberation was significantly correlated with the lipid level in the newly released larvae (Helm, Holland & Stephenson, 1973).