The direct piezoelectric effect in fine and coarse grain barium titanate ceramics is examined as a function of the external alternating and static pressure. It is found that domain wall vibrations contribute to the piezoelectric effect in both types of ceramics, but this contribution is smaller in the fine grain material. The external static pressure reduces domain wall contributions in the coarse grain samples and has little effect on the properties of the fine grain material. The results are consistent with the presence of a strong internal pressure in the fine grain ceramics. (C) 1996 American Institute of Physics.