an experiment was carried out to study the effects of whey in post-weaning diets of the piglets. forty-eighth piglets were supplied with four levels of volumetric substitutions of whey rations (0, 7, 14, 21%). the experimental design was a complete random block with four replications. animal performance and economic responses of piglets fed with whey were evaluated. piglets fed with control diet and 7% of whey had a higher feed intake (p<0.05) than other groups of piglets from 51 to 65 days. however, piglets fed with whey diets had a higher feed conversion (p<0.05) than those fed with control diets. there were no differences (p>0.05) among piglet groups fed with different diets for daily average gain of weight. if feed cost is greater than whey transport cost, whey diets can be an economical alternative to feed piglets in post-weaning period. in conclusion, diet substitutions until 21% of whey increases feed conversion of piglets.