If the main steam line from a nuclear steam generator were to break, the water in the steam generator would rapidly flash off in what is called a blowdown. Such an event could produce significant loading on the steam generator tubes, which if fractured, could lead to a loss of radioactive materials from containment. Thus, knowing the tube loading during such an event is an important input for safe design. This paper presents the results of an experimental laboratory study of the transient tube loading during a simulated blowdown. The working fluid was R-134a and the sectional tube model was a normal triangular array with a pitch ratio of 1.36. Tests were conducted with various levels of liquid R-134a and various numbers of tube rows. The transient tube loading is explained in terms of the associated flow physics and the maximum load is compared with existing models for tube loading obtained under steady flow conditions.