- “This is called a 'neuralyzer'. A gift from some friends from out of town. The red eye here isolates and measures the electronic impulses in your brain. More specifically, the ones for memory.”
An electro bio-mechanical neural transmitting zero synapse repositioner, commonly referred to as a Neuralyzer is a top secret device used by the MiB. It has the ability to wipe the mind of anybody who sees the flash. Once people are flashed, they seem to enter a trance and freeze and their eyes lose focus. Once victims are neuralyzed, the agents must make a story to replace the erased minds. Neuralyzers are paramount to the MiB's operation and maintenance of secrecy.
A neuralyzer is a device varying in size, that wipes the memory of a target. The set length of memory erased can be changed using dials, and the effects can be reversed. In order for agents to not be harmed by the effects, they have "Ray-Ban" sunglasses, that reflect the rays. It is the standard equipment of MIB officers. After being neuralyzed, if the agent does not supply the new memory, or a group of people get neuralyzed, a special team is sent in to give memories, and change the enviornment to reflect these memories. The neuralyzer can also affect species other than humans, like some animals and alien organisms.  
In Men in Black 3, the 1969 variant of a Neuralyzer is shown to be roughly the size of a small bus. Similar in shape to the miniature Neuralyzer, the 1969 version is a cylindrical tube positioned in the centre of a room and maintained by three MIB officials. Once the witness has been placed inside the Neuralyzer, it begins to rotate at high speeds. The inside of the Neuralyzer is lined with segmented columns that pulse green when the device is primed. It can be deactivated remotely by the push of a button.
In addition to this, Agent K carries around with him a portable version of the Neuralyzer to Cape Canaveral in Florida. This device is seemingly powered by a unit on his belt, and makes a similar bleeping sound to a dial up connection. The specifics of how the portable Neuralyzer works in comparison with the modern day equivalent are unknown.
In the Comics
The device in the comic books was referred to as a 'neurolyser'. This is also where the concept of using Ray-Ban sunglasses was introduced. It is seen more in the shape of a flashlight than a cigar-like shape made popular by the films. It does not simply erase memory like the movie counterpart does. In the comic books, the neurolyser is a device used for manipulation and control. When neurolysed, a witness is simply hypnotized and does what ever Agent Kay or Jay wants them to do, such as provide information, believe a lie and/or pass that lie along as fact.
- Main article: De-Neuralyzer
The effects of neuralyzers can be reversed with a de-neuralyzer. The only two known owners of a deneuralizer is MiB Headquarters, and Jack Jeebs (although his is illegal, and does not work nearly as well).
A neuralyzer comes with three settings, which can be changed using dials, which in the first movie were days, months, and years, though the second film implies that setting for hours was added. These can be set accordingly to the amount of time needed to be forgotten.
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- In the entire first movie, Jay calls the neuralyzer the "flashy thing," and only uses it once at the very end.
- Ironically, in the second movie, he neuralizes many partners, as they do not live up to Kay.
- In Men in Black II the neuralyzer color changes from red to blue.
- Based on the trailer for Men in Black III, the neuralyzer design has changed slightly, and the flash is now changed to cyan.
- The Statue of Liberty has a neuralyzer inside the torch to wipe the memory of the entire population of New York City. In the TV show the giant neuralyzer was in the Empire State Building.
- Over the years of MiB, the neualyzer has changed design.
- Oddly in the animated show, at times K or J would supply a fake memory, THEN nuralize witnesses.
- In the animated show, it was stated that only human brains could be affected by the effects of the neralyzers.