An analysis is conducted on the port component of the United States grain export system. A transshipment model is utilized which covers both United States internal and foreign shipments of corn, soybeans, and wheat during the four quarters of a year. The model suggests that there will be quarter to quarter constraints on port capacity but that annual capacity is adequate. Through sensitivity analysis a number of key factors were found which influence the adequacy of the current port system. Port adequacy is found to depend not as much on export market location as it does on domestic transportation rates and policies.