In teleosts , ammonia is a respiratory gas excreted to the environment from deamination of protein. Ammonia is a toxicant , especially when present as unionized ammonia (UIA). In this study, the effects of UIA on the ventilation frequency (fV), heart rate (HR) and resting oxygen consumption (MO2 ) of adult Nile tilapia (O. niloticus) were investigated in a series of experiments conducted under different UIA levels at acute (1h) and prolonged (24h) exposure periods. For the acute exposure, 8 fish (497.16± 36.77g) were exposed to progressively increasing UIA concentrations of 0 (control), 0.004, 0.01 , 0.035, 0.061 and 0.097 mM , by injecting pre-determined doses of ammonium chloride (NH4 Cl) stock solution into the experimental water hourly . Measurements of MO2 , HR and fv of the individual fish were recorded after one hour of exposure to each UIA concentration . In the p rolonged exposure trial, 8 different fish (436.68 ±47.02g) fish were exposed to 0.007 mM- UIA for 24h and the same parameters as the acute exposure recorded. The results of the study showed a significant (p<0.05) progressive decline in MO2 , HR and fV in respo nse to increasing UIA levels (Fig 1) . The 24-h exposure also resulted in a reduc tion of MO2 and HR by 30% and fV by 22%. It is evident from this study that Nile tilapia respond to UIA exposure by a depression in metabolic rate and cardio-respiratory work. This is in contrast to what has been described for other species, and may imply the use of a sit-and-wait strategy.