We have expressed the 5-HT3 receptor with two transient expression sytems: the Semliki Forest virus and the HEK293EBNA. Both systems allow the rapid expression of functional receptor protein at very high level: up to 107 receptors per cell. Large scale production yielded 1 to 2 mg of receptor protein per liter. The solubilisation of the receptor from membranes has been optimised; C12E9 solubilised 50 to 60 % of the ligand binding activity. A C-terminally engineered hexahistidine-tag facilited efficient purification of the receptor in a functional form. Sufficient quantities of protein are now available to enable biophysical characterisation of the 5-HT3 receptor.