Unsymmetric perturbations were introduced into the core of a large, critical, heavy-water-moderated, multiregional reactor. The resulting three-dimensional changes in flux level and shape with time were measured. Perturbations included: 1. Free-fall insertion of rods near the reactor center. Each rod contained 235U slugs in the bottom half and lithium slugs in the top half 2. Free-fall insertion of rods into an off-center radial position. Each rod contained 235U slugs in the bottom half and aluminum in the top half. 3. Withdrawal of cadmium control rods from the central 20% of the reactor core. Flux tilts calculated with the TRIMHX code were within 5% of measured flux tilts. TRIMHX provides a three-dimensional (hex-z geometry) solution of the few-group neutron diffusion and delayed precursor equations without feedback. Inputs to the calculations are available in sufficient detail to allow other methods of solution to be tested.