On April 16, 1997, at 2110 hours Pacific daylight time, the captain on United Airlines Flight 2327 was exposed to a laser beam during a descent into Ontario, California. The aircraft, a Boeing 737-500, N930UA, was operated by United Airlines as a regularly scheduled domestic passenger flight under 14 CFR Part 121 when the incident occurred. The flight originated in San Francisco, California, at 2029 on the evening of the incident. Visual meteorological conditions prevailed at the assigned altitude and an IFR flight plan was filed. The aircraft was at 13,000 feet msl about 5 DME from Hitop intersection on the Ziggy 3 arrival when the incident occurred. The captain was on the controls when he noticed a green light illuminating the aircraft. It appeared to him that the light was tracking the aircraft, but as he was pointing out that fact to the first officer, the light disappeared. He reported that, although his exposure to the light caused a minimal yet persistent loss of night vision, he was able to maintain control of the aircraft throughout the remainder of the flight. He told air traffic control (ATC) that the source of the light seemed to emanate from the western edge of the city of Rialto. He was advised that there were no reports of laser activity in the area. After landing, he notified United Airlines dispatch of the incident. The captain was subsequently examined by the staff at the Visual Psychophysics Branch, Armstrong Laboratory, Brooks AFB, Texas. Their examination revealed no evidence of permanent effects from the exposure. Investigators from the FDA, FBI, and the San Bernardino County Sheriff's Aero Bureau were notified; however, the source of the light remains undetermined. According to the FAA, there were no NOTAMs in effect for laser light activity in the Ontario area at the time of the incident.