Acute and sub-acute studies of profenofos [ O-(4-bromo-2-chlorophenyl)- O-ethyl- S-propyl phosphorothioate] on fish, Oreochromis (Tilapia) mossambicus, were carried out to assess the toxicity in relation to behaviour, morphology, and interactions with the targeted enzyme acetylcholinesterase (AChE, EC 3.1.1.7). Profenofos can be rated as highly toxic to O. mossambicus, with a median lethal concentration (LC(50)) of 0.272+/-0.0177 mg/l. The inhibitory and recovery pattern of brain and gill AChE was studied in vivo after exposure to a single LC(50) and multiple exposures to sub-lethal concentrations (0.108 mg/l) for 28 days, respectively. The LC(50)-exposed fish exhibited 90% inhibition of AChE activity in brain and gill in 24 h, and completely recovered within 23 days. Electron microscopy studies revealed an abnormal gill morphology, with distinct breakages in gill arches and rakers, along with deep lesions and erosions in the epithelium. Prolonged exposure at 0.108 mg/l also had similar effects, such as gill damage and AChE inhibition. The in vitro AChE study indicated that profenofos is neurotoxic and that it alters the apparent K(m) values widely in a concentration-dependent manner, resulting in a competitive type of inhibition. Based on the K(i) values, the sensitivity of AChE in brain was greater than that in gill tissue, at 2.38 x 10(-5) and 4.62 x 10(-5) M, respectively. The bioaccumulation values in head, body and viscera were estimated at regular intervals by gas chromatography method. The results indicated that the accumulation of profenofos was the highest in viscera followed by head and body, with depuration rates of 6.14, 0.16 and 0.12 micro g/h, respectively.