To determine the influence of collagen orientation on the magnetic resonance (MR) imaging appearance of articular cartilage, spin-echo MR images of normal bovine patellar specimens were obtained with the specimens rotated in 5 degrees increments between +75 degrees and -130 degrees. Hyperintense superficial, hypointense middle, and intermediate-intensity deep laminae were observed. Results of polarized light microscopy of histologic specimens confirmed the three zones, and transmission electron microscopy showed different collagen arrangements in the zones. An anisotropic effect of rotation on signal intensity was evident, especially in the hypointense second lamina. Because of the preferential alignment of water molecules associated with collagen, angular rotation of the cartilage in the direction of minimum dipolar coupling (55 degrees to the magnetic field) caused the cartilage to have a homogeneous appearance. The MR imaging appearance of these layers is strongly influenced by an anisotropic arrangement of the collagen fibers and by the alignment of the specimen relative to the magnetic field.