We present the experimental determination of the ion temperature in a neon-puff Z pinch. The diagnostic method is based on the effect of ion coupling on the Stark line shapes. It was found, in a profoundly explicit way, that at stagnation the ion thermal energy is small compared to the imploding-plasma kinetic energy, where most of the latter is converted to hydromotion. The method here described can be applied to other highly nonuniform and transient high-energy-density plasmas.