A mechanism for aligning a hypodermic needle or other probe with a predetermined location within a patient has a radio-transparent guide with a lengthwise slot for accommodating the needle. A pair of radiopaque rings spaced at opposite ends of the guide enables the surgeon to align the needle with the predetermined location and advance the needle in a continuous and steady movement toward the predetermined location in spite of the intermittent activation and deactivation of a two-dimensional x-ray apparatus. A slot in each of the rings, moreover, forming gaps in alignment with the lengthwise guide slot permit the guide to be removed as the needle approaches the predetermined location.
