A De-Neuralyzer is a special device that reverses the effects of a Neuralyzer. Although a neuralyzer is operated by a small flash of light, a de-neuralyzer is much more complicated. There are only two known De-Neuralyzers in existence. The first is the one in the MiB, and the second is in the basement of Jeebs' pawn shop. Jeebs' De-Neuralyzer was taken off of the Internet, and was even more complicated. Jeebs' machine acted like Rube Goldberg Machine, and even after that, the effects were not instant. With Jeebs' machine, it takes several minutes for a subject's memory to return. According to Jeebs, this is because he doesn't have updated software for the device that otherwise would allow immediate return.
Agent J had originally planned to use the De-Neuralyzer at the MiB to restore Agent K's memory in order to get information on where the actual Light of Zartha was thanks to Serleena's return. However, because of Serleena's takeover of the MiB and it instigating a lockdown before J could get a chance to activate the machine, they were forced to use Jeebs' pirated version to restore his memory. However, as K had previously neuralyzed the memory for the events of the Light of Zartha operation while he was still an agent, they could not restore that memory even with the de-neuralyzer.