A new method is presented for rendering general FRep (functionally represented) models using GPU-accelerated ray casting. We use the GPU acceleration for all computations in the rendering algorithm: ray-surface intersection calculation, function evaluation, and normal vector computation. Performing geometric intersection calculations in parallel with shading allows us to combine the whole process of rendering within one fragment program on GPU. The algorithm is well-suited for modern GPU and provides good performance with good quality of results; it is practically memoryless and does not require a powerful CPU.