Approximately 17:00 PDT, a Beech C33, N1310A, experienced a landing gear collapse or inadvertent retraction while performing a touch-and-go landing on runway 30 at Portland-Hillsboro Airport and the aircraft slid to a stop on its belly. The pilot and his passenger were uninjured. The pilot said he lowered the gear on downwind at midfield, and rechecked it on base and final. He said the approach and touchdown were normal but that, while rolling on the runway just after touchdown, he saw the nose start to go down. The pilot pulled back on the yoke but was going too slow to raise the nose. Inspectors found overstress damage to the nose landing gear linkage. In a post-accident test, investigators were able to extend the main landing gear manually but not the nose gear. In addition, the pilot did not meet the recency of experience requirements for carrying passengers.