On 22 January 1968, "Biddle" put to sea bound ultimately for the combat zone off the coast of Vietnam. Along the way, she transited the Panama Canal and made stops at Pearl Harbor and Guam before reaching her base of operations at Subic Bay in the Philippines on 24 February. After an availability, the guided-missile frigate departed Subic Bay for Vietnamese waters on 3 March. She entered port at Danang, South Vietnam, on 5 March and, the following day, was on her way for a PIRAZ (Positive Identification Radar Advisory Zone) station. For the next four months, "Biddle" alternated between periods at sea—either carrying out PIRAZ duty, serving as an antiair warfare (AAW) picket, or acting as a sea-air rescue (SAR) vessel—with time in port at Subic Bay or Yokosuka, Japan.