However—and this is important—the Prius won't let you drive the main battery down to absolutely no charge. When the battery becomes discharged to a certain point, the car is programmed to fire up the gasoline engine, which normally would recharge the battery as it propels the car. If you're out of gas, though, the Prius will try (and fail) three times to start the engine, and then it will go totally dead. Once this happens, a fault code must be reset in the car's engine computer before it can be started again. So even if you put gas in the tank, the car would need a technician to reset the code before you're on your way.
There's another worry about running your Prius out of both gas and juice: towing it. Since there is no conventional neutral in the transmission, pushing a dead Prius will be more difficult because the electric motor/generator system is also being turned. That leads to a bit of a problem in some hybrids: If they are flat-towed by a tow strap, they could generate electricity that could overheat the motor/generator. Toyota says its Prius, and other hybrids, will not generate electricity unless they've been turned on, which requires a charged 12-volt battery. Remember, the Toyota system has two batteries: a conventional 12-volt battery used only to start the system, and the main NiMH (nickel metal hydride) battery. If that 12-volt battery is dead, the Prius won't start, even if the main battery is fully charged. Fortunately, even though the 12-volt batteries in the Toyota and Lexus hybrids are located in the rear of the cars, there are posts in the engine compartment that allow you to "jump" the battery if it's dead.
The worst consequence of a tapped-out Prius would be if the depleted nickel-metal-hydride propulsion battery sinks too low in its charge that the engine and motor/generator cannot charge it. In this case, the battery would have to be charged by a Frankenstein machine that exists only at Toyota dealers. Toyota says that no dealers have ever reported needing to use the machine since the Prius's 2001 introduction in the U.S. Still, you don't want to be the first.