We describe a 3-D user interface for preoperative neurosurgical planning based on the physical manipulation of familiar real-world objects in free space. Using these passive interface props, neurosurgeons can apply their existing skills to specify spatial relationships in a natural and direct manner. The interface currently employs a head viewing prop, a cutting- plane selection prop, and a trajectory selection prop. Each prop is a simple real-world tool, the position and orientation of which is tracked by the computer. The behaviors associated with each prop serve as `interaction primitives' which can be composited to describe complex spatial relationships, resulting in a powerful, expressive, and conceptually simpler user interface. From the surgeon's perspective, the interface is analogous to holding a miniature skull which can be `sliced' and `pointed to' using the cutting-plane and trajectory props. Our informal evaluation sessions have shown that with a cursory introduction, neurosurgeons who have never seen our interface can understand and use it without training.