Redlight is known to increase the growth potential in the embryo of photodormant lettuce seed, enabling it to overcome the resistance offered by the seed coats (particularly the endosperm) or by an osmotically active medium. Determinations of the water potential of lettuce embryos germinating in osmoticum. carried out with a modified gravimetric technique which eliminates errors intro, duced by solute penetration into cellular osmotic space, showed that the water potential of red-light-treated embryos was lower than that of dark-treated ones, the difference being equivalent to the potential of a 0.30 molal mannitol solution. The force necessary for the radicle to penetrate the seed coats, measured directly, was found to be equivalent to the osmotic potential of 0.16 to 0.38 molal mannitol. Thus red light, acting through phytochrome, induces in photodormant lettuce embryos a decrease in water potential which is equal to that which is required for germination. A mechanism for this phytochrome-induced decrease in water potential is discussed.