The comparative effects of organic rice and conventional rice on the protein efficiency ratio (PER) in rats were investigated by feeding 40 male Sprague-Dawley rats for four weeks with three experimental diets containing polished conventional rice (PCR), unpolished conventional rice (UCR), unpolished organic rice (UOR) and a control protein diet (casein) under standardised conditions. All diets were prepared according to AOAC guidelines. The results showed no statistically significant difference (P > 0.05) among the values of PER (2.75 ± 0.14 - 2.80 ± 0.09) in rats fed with diets containing PCR, UCR or UOR. Similar growth was also observed among the three groups fed with different experimental diets. Additionally, residues of pesticides, viz. carbofuran, methyl parathion, p-nitrophenol and -cyfluthrin, in rat blood and rice samples were determined using liquid chromatography–electrospray ionisation tandem mass spectrometry. Pesticide residues were not detected in all serum samples of experimental rats and only p-nitrophenol was found (8.23 ± 0.65 - 12.84 ± 2.58 mg/kg) in all samples of the cooked rice diets, indicating that organic rice produced similar effect as conventional rice on PER and growth in rats.