A radioimmunoassay (RIA) for oxytocin (OT) in urine is described. 125I-OT was prepared, and antibodies were raised in rabbits against OT coupled to bovine serumalbumine. This allowed us to set up of RIA for OT which limit of detection is 1.25 pg/tube (0.6 microU). The use of an extraction procedure using CG50 Amberlite is essential. The recovery after extraction reaches 70.5%. pH 5 is the optimum pH were urine samples must be stored. The superposition of the elution peak of endogenous OT on that of exogenous hormone is an argument in favour of the validity of such an extraction procedure. Daily urinary excretion of OT reaches 9.58 mU +/- 3.48 in 18 healthy young men.