Freshwater sediment from an Oregon lake was spiked with copper in the laboratory to study the effect of storage on copper toxicity. Peat moss was added to enhance the carbon content of a portion of the sediment. One-half of the sediment samples were stored at 5C while the other half were frozen (-20C). At intervals up to 25 weeks, samples were brought to 20C and 48-hour toxicity tests were conducted in 1-liter beakers (200 ml sediment, 800 ml water) using the cladoceran Daphnia magna Straus. Copper released into the water from the sediment stored at 5C increased over the first 8 weeks of storage, then decreased. Freezing the sediment attenuated the release of total soluble copper into the overlying water, resulting in low or no toxicity to daphnids. Freezing is not recommended as a storage procedure because of its undependability in preserving sediment integrity for toxicity tests.