In this reprint of a seminal article, once considered quite controversial, the authors discuss their radical claim that rats laugh. Even more provocative, the authors found that this rat-joy sound, especially evident during play, could be amplified dramatically by what they formally call heterospecific (cross-species) handplay (tickling). The authors tickled rats during the most playful juvenile period of their development (older rats sometimes resisted tickling), then studied the ultrasonic chirping the rats produced in response. When analyzed, these vocalizations, occurring during playful rough-and-tumble bouts, suggested analogies to human laughter. The authors also found that fear inhibits playfulness and precludes laughter in rats: sudden, startling bright lights and rough handling reduces the chirping, but even more than these, the smell of a predator’s urine suppresses rat “laughter.” The similar positive emotional responses evident in humans and rats suggests a shared brain anatomy and similar neurochemistry, which, in turn, suggests new ways to investigate the ancient origin of human laughter. This article was the first publication to summarize the full set of groundbreaking experiments that changed the way many researchers and scholars consider animal feelings, human nature, and the field of play.