Plant water deficits during flowering and early kernel growth reduce the yield potential of maize (Zea mays L.) by decreasing kernel number per ear. Our objective was to determine if zygotic abortion, which increases in water‐deficient plants, is caused by a decrease in assimilate supply to the ear. Plants were grown in soil/sand media in a controlled environment. Beginning at silk emergence and continuing for 3 wk after pollination, leaf photosynthesis was inhibited ≈50% (moderate) or 100% (severe) by withholding water (water‐deficit treatment) or decreasing light intensity (low‐light treatment). Silk water potential (ψw) at pollination was about −0.7 and −1.1 MPa in the moderate and severe water‐deficit treatments, compared to about −0.4 MPa in well‐watered plants. Mean kernel number per ear was 581 in control plants and was decreased 48 and 99% by the moderate and severe water‐deficit treatments, respectively. In the corresponding low‐light treatments, kernel number was decreased 40 and 100%. Thus, losses in kernel number pe ear reflected the relative inhibition of photosynthesis during early kernel growth, regardless of plant water status. Compared to controls, the water‐deficit and low‐light treatments decreased partitioning of dry matter, carbohydrates, and reduced N to the kernels. However, carbohydrates continued to accumulate immediately after pollination in the stalk and leaf tissue. This suggests that some stalk and leaf reserves were unavailable to support early kernel growth at low ψw. Collectively, these data indicate that early kernel development is highly dependent on a continued supply of carbohydrates from concurrent photosynthesis. Water deficits increase the probability of developmental failure by limiting this supply.