Acetylcholine (ACh) is a chemical transmitter serving to propagate an electrical perturbation across the synaptic junctions of animals. ACh and AChE have previously been demonstrated to occur in plants. In this work, we detected AChE at the interface between stele and cortex of the mesocotyl of Zea mays by measuring hydrolysis of acetylthiocholine and by liberation of labeled acetate from [1-(14)C]ACh. AChE activity was also detected in a crude membrane fraction. The hydrolytic activity is inhibited by neostigmine. Hydrolysis of ACh was also measured after injection of [1-(14)C]ACh into kernels of Zea mays and the radioactivity transported into the mesocotyl cortex. A gravity stimulus was then given by placing the plants in a horizontal position. Significantly more radioactivity was found in the lower cortex of horizontally placed seedlings. A working hypothesis is presented for the involvement of ACh and AChE in the tropic response of Z. mays seedlings.