Although the nitrogenous excretory products of a number of marine bivalve molluscs have been identified (Hammen, Miller & Geer, 1966; Hammen, 1968; Emerson, 1969; Allen & Garrett, 1971; Bayne, 1973 a) and average values for rates of nitrogen excretion recorded for some species (Lum & Hammen, 1964; Hammen et al. 1966; Bayne, 1973a), there is little information available on the ways in which environmental factors may affect either the balance between the various nitrogenous end-products, or their rates of excretion. The most thorough investigations have dealt with the effects of reduced salinity, which causes an increase in the rates of excretion of ammonia by Macoma inconspicua (Emerson, 1969), Mya arenaria (Allen & Garrett, 1971) and Mytilus edulis (Bayne, 1975). Dilution of the medium is also known to accelerate the loss of amino acids from the body, as part of the mechanism of volume regulation (Pierce & Greenberg, 1972, 1973). There is some evidence of altered rates of excretion during starvation (Hammen, 1968; Emerson, 1969; Bayne, 1973 a, b) and differences in excretion rate due to differences in animal size have been documented for Donax vittatus by Ansell & Sivadas (1973) and for Mytilus californianus by Bayne, Bayne, Carefoot & Thompson (1976 a, b).