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Organ samples were collected from plaice, Pleuronectes platessa L., captured in the highly oiled Aber Wrac'h and Aber Benoit at five intervals during the period 1978-1980, following the Amoco Cadiz crude oil spill. Reference plaice were obtained along the western and southern coasts of Brittany. Tissue samples were fixed, processed and stained by routine histoiogic procedures for qualitative and quantitative histopathological examination. The predominant lesions and conditions which characterized fish from the oiled estuaries were: fin and tail necrosis, hyperplasia and hypertrophy of gill lamellar mucous cells, gastric gland degeneration, decreased hepatocellular vacuolation (lipid), increased concentration of hepatic macrophage centres and lateral trunk muscle fibre degeneration. Dilation of Bowman's space, glomerular hypertrophy and abdominal muscle fibre degeneration were pronounced at the last sampling interval. Other types of lesions were seen at a lower frequency: hyperplasia and fusion of gill lamellar epithelium, telangiectasis of gill capillaries, hepatopancreatic degeneration and hepatic necrosis. Fish muscle and liver collected for biochemical analysis to detect the presence or absence of petroleum hydrocarbons were negative for Amoco Cadiz oil fractions. At the same timeoysters, Crassostrea gigas Thunberg, collected from theoiled estuaries throughout the study period, showed high levels of Amoco Cadiz oil fractions. These data, along with the type and frequency of lesions observed, suggested that the plaice had been chronically-exposed to petroleum hydrocarbons of Amoco Cadiz origin. Some relief from chronic pollutant exposure was indicated by a decrease in gill mucous cell concentration at the last sampling interval. An increase in abdominal muscle fibre degeneration and an increase in the frequency of dilation of Bowman's space with glomerular hypertrophy at the last sampling interval suggested a latent response to the polluting agent.