The purpose of this research was to investigate the possible variations to the pharmacokinetics of nimesulide by the effect of age using an animal model. An experimental, analytical, prospective and longitudinal study in five dogs, from birth to 730 days of age was carried out. Nimesulide blood levels were measured in different months; concentrations were determined by HPLC-UV. Pharmacokinetic parameters were calculated by using the WinNonlin software. There were statistically significant differences (p <0.05) in most of the pharmacokinetic parameters between study of 6 months against the other three studies in different ages. Changes in the pharmacokinetic parameters of nimesulide as a result of age, are determined by the growing and maturation of the animals. Resulting data suggest that nimesulide can be used safely as a long-term analgesic in dogs, but, the dosing regimens in humans should be different when administered at early age.