This paper analyses the imagery on two different Zapotec ceramic forms: an open-ended cylinder and an effigy vessel, both from the Royal Ontario Museum in Toronto. In this study, I propose that the figures on these objects represent impersonators of the Zapotec storm god Cocijo. The impersonators would probably have been rulers playing the role of this god and are carrying out a ritual associated with the agricultural cycle of corn. A comparative method that combines historical archaeology, ethnography, and iconographic analysis reveals clues to the function and significance of the vessels. The study leads to the conjecture that these objects were used in connection with blood offerings during corn-harvest rituals. These conclusions address the nature of ancient Zapotec religion and cosmology and provide evidence that the Zapotec performed rain and fertility rituals associated with the corn harvest similar to those of other cultural groups in Mesoamerica.