Were dinosaurs warm-blooded, cold-blooded, or neither?

Probably neither, in the simple sense. Today it is generally thought that many or perhaps all dinosaurs had higher metabolic rates than living reptiles, but that the situation is more complex and varied than Bob Bakker first proposed. Non-avian dinosaurs lasted about 150 million years, so different groups very likely evolved different metabolisms.

The question is slippery because 'warm-blooded' is a complex and ambiguous term that includes some or all of several traits, such as keeping a fairly constant body temperature, keeping a high metabolic rate at rest, and generating heat internally. In the early years of dinosaur science they were seen as sluggish, sprawling cold-blooded lizards. From 1968 Bakker argued strenuously that dinosaurs were warm-blooded, active animals.

The clues pull in different directions. Generating heat internally is expensive: modern mammals need 10 to 13 times as much food as modern reptiles. Yet very large animals can be steady without that cost, since their thermal capacity is so high that their temperature takes two days or more to change significantly. On the other side, the Alaska North Slope has no fossils of large cold-blooded animals such as lizards and crocodilians, which were common then in Alberta, Montana and Wyoming. That suggests that at least some dinosaurs were warm-blooded.

Some of Bakker's own arguments did not survive. His predator-prey argument was criticized on several grounds and is no longer taken seriously, and bone structure turned out not to be a reliable indicator of metabolism.