Chlorpyrifos (CPF) is a widely used organophosphate insecticide. Japanese medaka, Oryzias latipes was selected to investigate the effects of chlorpyrifos on survival rate, hatching rate, reproductivity and histopathological changes. Adult fish were exposed to CPF at concentrations of 20, 40, 80 and 160 ㎍/L, and the test duration was 4 weeks. An experiment with eggs was also performed under CPF exposure continued for 7 days covering from the fecundation to end-hatching stages. Survival rate of the adult medaka decreased in CPF 80 ㎍/L and 160 ㎍/L treatment groups compared with control, solvent control, 20 ㎍/L or 40 ㎍/L treatment group. Spontaneous start of feeding significantly decreased compared with the control (p＜0.01) when exposed to CPF at above 40 ㎍/L. Total spawning decreased by the 20 or 40 ㎍/L CPF treatment compared with the control or solvent control in the test with eggs obtained from untreated adult medaka, and abnormal eggs increased in those CPF-treated groups. There were certain adverse effects at above 40 ㎍/L determined from liver and gonad histological examinations. The results indicate that CPF has insignificant toxic effects in Japanese medaka at less than 20 ㎍/L in long term exposure.
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