In 2265, she was assigned as a medical officer of the cultural observation team sent to planet NGC 667 to study the Grennai. Less than a year later, the mission was abandoned, in part due to the danger posed by atmospheric radiation to the non-native beings, and also because of Prime Directive violations by fellow team members.
In 2268, Starfleet sent the USS Enterprise, where her old friend McCoy served as chief medical officer, to return Jendra to Earth. Instead, Captain Kirk helped to fake Jendra's death, allowing her to continue her work with the Grennai. (TOS short story: "First, Do No Harm")