What's the best way to get from Venice to Zagreb?
Tom many rental car companies do not allow you to take cars from Italy into former Yugoslav countries. I don't know if this has changed but in the summer of 2005 we visited Croatia and Slovenia ending our trip in Venice. We were thinking of starting in Venice but because of the rental car situation, we flew into Dubrovnik where a car would have been a major headache. We took the bus from Dubrovnik to Split (Dalmation coast is stunning!) where we picked up our rental car when ready to leave Split as we did not need a car here either. I checked the Trenitalia's website and it appears that there is only a Euronight train to Zagreb, leaving Venice at 21:20 and arriving in Zagreb at 4:18, journey length 6 hrs. 58 minutes. Hope this info helps. We loved Croatia and Slovenia!
Train or train/bus combination is possible, but it's a long trip - about 9 hours. The Deutsche Bahn website shows a couple more options than the Trenitalia website. An option going through Villach would be best. Driving would likely be a better option, provided you can get around the restrictions Janet mentioned and can negotiate a reasonable drop-off fee. To fly, you would probably need to transfer in Rome or Frankfurt. Unfortunately, none of the Venice to Zagreb options are really great.