This study aimed to determine the toxicological effects of cadmium, lead and mercury present in the waters of Guiguinto River on Tilapia nilotica. Fifteen aquaria were used, each seeded with 10 pieces of Tilapia nilotica and exposed to cadmium, lead, mercury in increasing concentrations. The control tanks had the same concentration with the river that was earlier found to be contaminated with cadmium, lead and mercury. Trials were done in duplicate for a period of two weeks. Daily observations were done for the physico-chemical characteristics. Fishes that died were brought to the laboratory for AAS analysis, and for tissue processing for histopathology examinations. Results showed that at increasing concentrations of different toxicants, degenerative changes on the tissues of the brain, gills, muscles and stomach occurred, causing physiological behavioral changes on the fishes. Mercury was the most toxic since 50% of the test organisms died within 6 to 7 hours of exposure at concentrations of 1, 2 and 5 mg/L. The Pearson Product of Moment Correlation showed that there was a significant relationship between the exposure time and the various concentrations of cadmium, lead and mercury so as with the bioconcentration factor.