Juveniles of Prochilodus lineatus were exposed to sediments collected from one of five sites along an urban stream into which various types of contaminants are discharged. After 24 or 96 h fish were examined and the results compared with those from control groups (fish exposed only to water, for the same period). Plasma ion levels varied significantly and fish exposed to site 5 sediment showed a transient increase in both sodium and chloride concentrations. Plasma glucose was significantly higher in fish exposed to sediment from sites 2 and 5. The higher liver glutathione-S-transferase activity registered in fish exposed to sediment from sites 1, 4 and 5 suggests the presence of organic contaminants at these sites and the enhancement of liver catalase activity in fish exposed to sediment from sites 3 and 4 may be due to contaminant-mediated oxyradical production. The overall results revealed that sites 4 and 5 are more severely contaminated, probably due to organic contaminants from agricultural sources and municipal landfill.