1. Guppies (Poecilia reticulata) were exposed for 96 h to 4-chloroaniline (4CA), 3-chloroaniline (3CA) and 2-chloroaniline (2CA) in a static exposure system. 4-Chloroacetanilide (N4CA), 3-chloroacetanilide (N3CA), and 2-chloroacetanilide (N2CA) were identified in test water as the respective biotransformation products. 2. Dependence of biotransformation on aqueous concentration was investigated with 4CA. The measured biotransformation product in water (N4CA) increased with concentration increase of parent compound, 4CA, and with exposure time. 3. Quantitative differences in net acetylation of the three monochlorinated anilines were examined. The amount of acetanilide found in water increased in the following order: N2CA < N3CA < N4CA. Mass balance calculations indicated the highest loss of aniline occurred for 2CA. This compound, which is highest in toxicity, had the smallest amount of acetylated product formed. 4. This study proves, for the first time, that the acetylation reaction in fish is reversible. Quantitative differences in the net deacetylation of monochloroacetanilides were observed. The amount of deacetylated product found in water increased in the order 4CA < 3CA < 2CA.
