Forty-five boars, 45 barrows and 45 gilts were randomly allotted in two experiments to dietary zinc levels varying from 22 to 80 ppm. Dietary calcium was maintained at 0.7% of the diet. All pigs were fed and watered ad libitum. Blood samples were withdrawn from the anterior vena cava on four occasions (initially, 4 and 8 weeks and upon completion of the experiment) for serum alkaline phosphatase and zinc determinations. Inadequate dietary zinc caused significant reductions in daily gain, daily feed intake, serum zinc concentration and alkaline phosphatase activity in all sexes. Zinc deficiency symptoms including reduced gains, feed consumption, serum zinc concentration and alkaline phosphatase activity and the appearance of parakeratosis were more severe in the boars and gilts than the barrows. Parakeratosis was not observed in barrows fed diets containing less than 30 ppm of zinc while 33.3% of the boars and 26.7% of the gilts fed equivalent diets exhibited parakeratotic lesions.