Exhaust smells

I have noticed that in the last two days my exhaust smells horrible if I let the car idle for a couple of minutes. I started it this morning before going to school, and when I went out to leave about 10 minutes later, everywhere within 10 feet of the car was horrible. I almost gagged because it was so bad. What could be my problem?
Thanks
Tim

[WANT '71 280SEL]

'72's don't have cats. If it smelled something like rotten eggs it could be running rich and need maybe a fuel pressure regulator? Peter would be the person to ask. Werner told me that rotten egg smell from my dad's would probably be a FP regulator, so I don't know about yours.
-David

[300SDog]

Has the car been off the road for awhile?? ........mice can crawl into exh pipe then dying - am not joking - cure of mouse problem is placing cut tennis ball over exhaust pipe if car goes off the road.

I drive my car at least 3 times a day. I don't know if the car is running rich because it doesnt seem like it has that much more power.
Tim

[Tomguy]

When it's cold it runs VERY rich - to the point of black smoke!
If it's cold when you notice this smell, that's probably why!

The weather is actually starting to warm up here a bit, but I have not smelled this at all during this winter, and it has gotten VERY cold.
Tim