Between 7 and 9 postnatal days, the kidney, spleen heart, and liver from vitamin B6-deficient young either plateaued and failed to gain weight or lost weight, whereas brain continued to gain weight but at a more reduced rate than normal. The level of cofactor in kidney of experimental animals was below that of controls at all time points studied. alpha-L-Fucosidase activity was already moderately increased in deficient kidney at 3 postnatal days and remained elevated throughout the time course. beta-D-galactosidase, acid phosphatase, and hexosaminidases A and B in vitamin B6-deficient young all showed moderate increases in activities after 9 days, at one or more time points. In brain, alpha-L-fucosidase activity was increased somewhat throughout the time course, but changes in beta-D-glucuronidase, beta-D-galactosidase, alpha-D-mannosidase, acid phosphatase, and hexosaminidases A and B did not occur until after 7 postnatal days. When weights of kidney and brain were both normal, there was a marked elevation in alpha-L-fucosidase activity in these organs suggesting that this increase in enzyme activity may be a specific effect of vitamin B6 deficiency. Elevated alpha-L-fucosidase activity in kidhney may reflect alterations in fucose-containing glycosubstances of glomerular basement membranes.