A technique based on the optical Schlieren method is developed to investigate in situ the growing crystal morphology under real rapid profiling growth conditions. The paper deals with the technique and results of experiments related to rapid profiled growth of a KDP crystal with the cross-section 8 x 8 cm(2) and the (101) or (100) face growing at a normal growth rate upto 1 mm/h. It is shown that the structure of growth centers on the growing surface essentially depends on the growth conditions, which provide additional possibilities of the crystal quality control.