Cycloheximide and streptovitacin A administered in vivo to rats display a similar dual effect on the labelling of soluble liver proteins by valine-14C, and result in a similar enhancement of liver uridine kinase activity. On the other hand, in pylorus-ligated rats, both antibiotics markedly depress gastric secretion, acid output, and the level of mucoproteins and proteolytic activity in secreted juice. Streptovitacin A on a molar basis was in all cases 5 approximately 8 times more effective than cycloheximide.