My '94 Mazda Miata is losing antifreeze at an alarming rate, a gallon a day. It seems to be worse when the heater is on. I cannot tell where the leak is and there are no puddles under the car. The oil is not milky looking and there is no condensation in the oil fill cap. Any help would be appreciated.:confused:

Check the heater core. Do you smell anti freeze inside the car?

Are you having any drivability issues? Like rough running?
If there are no apparent leaks, that could indicate the coolant is burning inside the engine.
If you don't think it's that, try by-passing the heater core and check again for coolant loss.