Urethral play is play that involves inserting objects into the urethra, and sometimes the bladder. In some cases, these scenes can be associated with people into medical scenes (playing amateur urologist hour), or in the BDSM scene, is closely associated with control and dominance. For the submissive/bottom, the psychological idea of being violated in such a manner, the inability to control bladder functions (if catheters are inserted into the bladder) along with the physiological sensitivity of the urethral lining can be quite a trip. For most people, the idea of urethral play is repulsive and dangerous, from my experience I find this to be untrue and wish to share some advice, opinions and suggestions.
In urethral play, you generally have two kinds of instruments that you can use. A sound is one type of device that comes in a variety of widths and is inserted into the shaft. Most people who are heavily into the use of sounds usually try to dilate or stretch the urethra, although there are many who get into seeing how far they can go as well. Sounds are sort of like wands usually made out of surgical stainless steel. Catheters are very different. A catheter is inserted into the shaft with the objective of reaching the bladder. Catheters are much longer and are usually made out of latex or plastic. Some types of catheters, like a Foley, have a small balloon that is inflated after it reaches the bladder. Once the catheter is in, the person who has it in pretty much loses control of when they can piss. There are many different type of both sounds and catheters. Many of the sounds are curved to follow the natural shape of the penis, while some of them are straight and forces the urethra to follow the shape.
* Do not use or take drugs that may deaden any sensations. Injecting lydocaine or any other topical anesthetic into the shaft is dangerous and won't allow you to tell your limits (especially if your goal is to stretch out your urethra).
* Use water-based sterile lube to aid insertion of objects. Do not use lubes that contains nonoxynol-9 or any flavored lubes, both will irritate the urethra. Oil-based lubes are probably not the lube of choice, especially with anything latex, and also is more difficult to expel when you're done.
* Be sure everything you use is clean and if possible, sterile... this goes more of less for sounds and other items inserted only into the urethra, not the bladder. Never use an unsterilized catheter or anything unsterilized if you plan on getting into the bladder.
* Encourage pissing after you're done, this will help to clean out residues, dirt and any other small bacteria that may have come in from the outside.
* Even in hospital conditions infection is not uncommon. Should you get an infection, check it out immediately.
* In any BDSM, if you don't trust the person doing it, don't do it.
* Never flush or force any liquids into the bladder.
* Be careful not to let go of the sound when it's inserted, once the sound is inserted more than 4 inches, the sound will usually rotate slightly and pull the rest of the sound down. Should the sound be totally inserted into the urethra and you lose grip, don't panic and relax - the sound will come out naturally.
* Be aware that urethral play, no matter how carefully done, does irritate the urethra... so if you have any penetrative sex/oral sex afterwards, use a condom. (Actually, you should be using a condom all the time anyways...)