Thanks for your respones speedball. A few years back we had some work done to the upstairs bathroom and had the vent exstended down to the basement. It's a 3inch pvc pipe that is sticking down from the upstairs floor and about 15 to 20 feet from where we are going to add the toilet and shower. The water is already there from a past sink that someone had hooked a toilet up to a sink sized pipe which we now know was wrong and have to put the proper size line in for the toilet, so I guess what I am trying to ask, is do we run a line for the shower to the larger pipe which would be the toilet then run it to the main line, with the air vent coming in right at the(T). Or do we hook the airvent up to the new toilet pipe we are running to the main line, before we hook it up to the main line? Wow I wish I could draw a picture.. lol Thanks -Dan-

"Do we run a line for the shower to the larger pipe which would be the toilet then run it to the main line, with the air vent coming in right at the(T). or do we hook the airvent up to the new toilet pipe we are running to the main line, before we hook it up to the main line?"

If you can connect the shower to the vent before the toilet you can get away without having to run a separate vent for the shower. If not you will have to run a separate vent for the shower. If you would add a lavatory you could conne\ct the shower to the lavatory drain and be wet vented by it.
Here's a typical bathroom rough in. Toilet connects to sewer main. The toilet wet vents through the lavatory vent. Lavatory connects to toilet drain and runs a vent out the roof or revents back into a dry vent in the attic off the top the stubout tee. The shower connects to the lavatory drain and is wet vented by it. This is a normal rough in and is acceptable both by local and state codes but also The Standard Plumbing Code Book. Good luck, Tom