l recently bought a 1993 Peugeot 306 diesel. The car is fine and runs great, but the water level in the radiator drops dramatically in just a couple of days. There are no signs of an external water leak, so where could it be going? l have to put about 2 to 3 pints of water in the radiator every three days or so.
EDIT
there is no sign of water in the oil or the exhaust fumes.

Could be the head gasket and water is leaking into the engine. Check the oil for a foamy brown appearance or white smoke from the exhaust.
Get it checked out if either are happening as it wo not just get better.

Hopefully it is an external leak (so slight that you have not spotted it yet)
failing that it is probably a head gasket and water is going out through the exhaust, keep an eye on it, (it may become more noticeable at a later time)

Leprechauns.

ethropians have been drinking it

sounds like head gasket, no sign of oil in the water dosn`t always mean its not, it may not have reached that stage yet

Well, someone could be correct when he refers to mixing water with the engine oil because of a bad head gasket but also could be that the car is getting too hot & the water just evaporates. It is not very recommended to use water in ur car is radiator instead u should use coolant because it has a higher boiling temperature & protects the water pump & other engine components. Also, the radiator could have a small hole where the water is escaping. Have someone to look under the car for a brownish spot in the bottom of the radiator & hoses. Change the water for coolant & do not let the temp needle pass over 105 degrees.

306 is are famous for the heater matrix splitting, its worth looking to see if the passenger footwell is wet? Thats generally where the water ends up. If it is, you will need to replace the heater matrix, unfortunately, expensive to have done, and a sod of a job to do yourself! Good luck