In 2266, Uhura, who had been assigned to serve as the Enterprise's communications officer, used an earpiece in her left ear to determine that the starship was not receiving any signals from an odd cube encountered in uncharted space.
After the Enterprise destroyed the cube, Spock wore an earpiece in his right ear while he reported to Captain Kirk that there was no sign of the cube, and considered possible consequences if the starship continued on its course.
While later wearing her earpiece, Uhura hailed an object approaching the Enterprise that was actually a ship under the command of Balok, who had meant the cube to be a warning buoy. Shortly after she contacted Balok's ship, relaying a message from Captain Kirk, navigator Dave Bailey put another earpiece to his right ear. He used the device to receive a message transmitted over his navigation beam. Uhura, still wearing her own earpiece, was able to play Balok's message aloud so that the other officers aboard could also hear what was being sent. Both Bailey and Uhura continued to hear the message through their earpieces but, when a piercing crackle of static was transmitted from Balok's ship, the two officers immediately pulled the devices from their ears, in pain due to the disturbingly loud noise.
Uhura continued to use her earpiece as she relayed messages from Captain Kirk to Balok, often removing it at her own accord. However, she twice removed the device from near her ear when, both times, the bridge began to resonate with a whining noise that steadily increased in pitch. She also later used the earpiece to detect a repeating distress signal from Balok - while he was in a smaller personal ship that had apparently lost power - to the craft he had departed, the Fesarius. (TOS: "The Corbomite Maneuver")
While the Enterprise pursued an unidentified vessel actually belonging to Harry Mudd, Uhura used her earpiece to ascertain that the occupants of the craft were either refusing to answer her hails or not receiving them. After the vessel entered an asteroid belt and began to superheat, Uhura heard a distress signal from the craft through her earpiece, but Harry Mudd and three female companions of his were rescued and beamed aboard the Enterprise shortly thereafter. Uhura later used her earpiece to arrange a meeting between the Enterprise's senior staff and two miners from Rigel XII, as the starship required replacement lithium crystals. While officers from the ship reluctantly arranged a trade in which they would receive the crystals if they provided the miners with Mudd's companions, one of the women and a pursuing miner became lost in a magnetic storm on the planet. As part of the subsequent search, Uhura used her earpiece to maintain communications with the miners, until the storm worsened and audio contact was consequently broken. Both the miner and the woman were ultimately recovered, although Uhura's earpiece was not responsible for locating them. (TOS: "Mudd's Women")