Adult rats fed proteins as a meal given during the daytime exhibit alterations of liver protein metabolism characterized by simultaneous stimulations of protein synthesis and degradation, particularly during the hours following protein ingestion. The purpose of the present work was to determine if the stimulation of liver protein breakdown could be related to biophysical alterations of the lysosomal system. There is a growing amount of evidence to suggest that the lysosomal vacuolar system is involved in the physiological regulation of overall proteolytic rate. Rats, trained to eat a protein meal 2 hrs after the onset of light, were killed 6, 9, 18, 21 and 24 hrs after protein intake. Three fractions were isolated from 0.25 M sucrose liver homogenates after differential centrifugation. The mitochondrial-lysosomal fraction was further analyzed by isopycnic centrifugation in sucrose gradients. Three specific lysosomal enzyme activities were assessed: N-acetyl-beta-D glucosaminidase (marker), cathepsin D and cathepsin C (proteolytic enzymes). Total activities remained unchanged at all time-points, but the distributions between the different fractions recovered after differential centrifugation were altered 6 and 9 hrs after protein intake. A significantly higher percentage of N-acetyl-beta-D-glucosaminidase, cathepsin D and cathepsin C activities were recovered in the M + L fraction, suggesting a shift towards lysosomal forms of lighter density. This was confirmed by density gradient analysis. Thus, even in adapted rats, acute administration of protein during the daytime quickly induced biophysical alterations in the lysosomal system. The lysosomal distribution pattern observed at 6 and 9 hrs after protein intake might be due to lysosome enlargement by active autophagy and/or by the sequestration of lighter cellular material.