Weanling and young adult rats were fed diets containing lactose (L) up to 50 g per 100 g diet for 15 days. The growth rats variation (W), fecal pH, net protein ratio (NPR) and protein efficiency ratio (PER) were determined. In weanling as well as in young adult rats an impairment of L was only detected for higher levels (greater than 20%), and diarrhea occurred only at 50%. The NPR and PER were partially effected by L because of the stimulation of appetite between 1 and 5 g%. The results for 10 and 20 g% of L were not different from controls. It was concluded that rats are not intolerant to levels of L less than or equal to 20%. The evaluation of protein quality in presence of L must be conducted with adapted methodology.