Labelled phosphate incorporation into RNA and C¹⁴-leucine incorporation into protein of Yoshida ascites hepatoma cells have been studied, in vitro, in the presence of DL-glyceraldehyde and DL-erythro, threo-$\alpha$, $\beta$-dihydroxybutyraldehyde. Both aldehydes decrease protein synthesis to a similar extent, whereas differential effects are seen on P³²-incorporation into RNA. Glyceraldehyde inhibits RNA synthesis after a lag period of 30 minutes, reaching inhibition values of approximately 30% at 120 minutes of incubation. Inhibition by dihydroxybutyraldehyde does not show lag period and reduces P³² incorporation into RNA to 50% of control values at 120 minutes of incubation.