Kernels at the ear tip of field grown maize (Zea mays L.) often abort during flowering, resulting in significant yield loss. The objective of this study was to determine if abortion is initiated by an inadequate supply of carbohydrates for growth of ear tip kernels, and/or by a hormonal signal. Field grown maize plants were either unshaded or shaded during flowering to increase kernel abortion. Nonstructural carbohydrates, indoleacetic acid (IAA), abscisic acid (ABA), and cytokinins were measured in aborting and nonaborting kernels, before and after abortion occurred. Kernel abortion was initiated 8 days after anthesis (DAA) and was complete by 12 DAA, when kernels ceased dry weight accumulation. Concentrations of reducing sugars, sucrose, and starch in aborting kernels were not significantly different from those in nonaborting kernels up to 12 DAA. Also, total carbohydrate concentrations were higher in the cob of aborting than of nonaborting kernels from 8 to 26 DAA. These data suggest that kernel abortion is not initiated by an inadequate supply of carbohydrates. However, accumulation of reducing sugars in the cob of aborting kernels suggests that transfer of sugars from cob to kernels is impaired early in the abortion process. Differences in IAA, ABA, and cytokinin concentrations between aborting and nonaborting kernels were only observed after abortion was complete. Kernel abortion is apparently not initiated by a signal of these hormones. After completion of abortion, aborted kernels contained higher concentrations of ABA and lower concentrations of IAA than nonaborted kernels. There was also a trend toward higher concentrations of zeatin riboside in nonaborted kernels. Abscisic acid may promote kernel abortion after the process has been initiated. A mechanism for kernel abortion is discussed.