Steel storage racks, made of cold-formed steel, are used extensively in industry for storing goods. Two main racking systems prevail, referred to as "selective" and "drive-in" racks. International racking design codes mainly deal with selective racks, while limited design guidelines are available for drive-in racks. Drive-in racks require minimum floor space by storing pallets one after the other with no space between them. The forklift truck drives into the rack to store the pallets on the first-in, last-out principle. To allow forklift passage, drive-in racks can only be braced at the back (spine bracing) and at the top (plan bracing) in the down-aisle direction resulting in a complex slender structure with poorly understood 3D behavior and increased risk of collapse. As yet, tests on drive-in rack systems to accurately capture their 3D behavior are not available in the literature. This paper presents experimental results from full-scale tests conducted on a complete drive-in rack system. Experimental investigations of the load transfer and relative stiffness under various horizontal loading conditions are presented. Experiments have been performed on loaded and unloaded racks.