The conditions near a crack in a plate, whether stationary or growing, are usually considered to be a state of plane stress. In this note, however, it is shown that appreciable normal stress through the thickness can occur when the crack is growing rapidly. This gives rise to a state of triaxial tension in the core of the plate near the head of the crack and can thus contribute to the brittle behavior observed in such cases. The amount of triaxiality is strongly dependent on the extent of the plastically deformed zone at the end of the crack. This fact may be one reason for the greater brittleness shown by thick plates.