One of only five times he actually used the "I'm a doctor, not a(n) ___!" phrase.
The funniest one is from the episode "Friday's Child", with guest star Julie Newmar playing the incredibly pregnant wife of a tribal leader on a planet the Federation is trying to negotiate trading rights with. Klingons are also involved, and in the debate and struggle for power, the leader is killed. The new leader orders Newmar and the unborn child killed, as they represent a challenge to his rule. However, Kirk, McCoy, and Spock grab her and flee into Vasquez Rocks the nearby quarry hills. As she is "pregnant", Newmar is having some difficulties in making the ascent. Kirk orders McCoy to assist her, leading to the line:
"Look, I'm a doctor, not an escalator!"