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The toxicity and histopathology of ivermectin was studied in 3 and 35 g sea bass Dicentrarchus labrax L. following in-feed, oral intubation and injection administration at dose rates ranging from 0.5 to 3.5 mg kg(-1). Estimated LD50 values for 3 g fish were 0.335 and 0.106 mg kg(-1) following oral intubation and injection administration respectively, for fish reared at 11 degrees C; and 0.839 and 1.023 mg kg(-1) following oral intubation and injection administration, respectively for fish reared at 20 degrees C. For 35 g fish reared at 11 degrees C, the estimated LD50 was 0.523 and 0.361 mg kg(-1) following oral intubation and injection administration respectively. No signs of toxicity were observed when the compound was administered via the feed at 0.5 and 0.7 mg kg(-1). However, toxicity (> 10%) was observed at dose rates of 0.2 mg kg(-1) and higher when the compound was administered via oral intubation and at 0.5 mg kg(-1) when administered via injection. The compound was significantly more toxic to fish reared at 11 degrees C than at 20 degrees C. Further, ivermectin was more toxic to 3 g than to 35 g sea bass when administered via injection. Histopathological examination of the major organs revealed pathology was largely restricted to gills and intestinal tissue. In 3 g sea bass, lesions were also found in the kidneys.