Hydrazine is a chemical of outmost importance in our society, either for organic synthesis or energy purpose. The direct conversion of NH3 to hydrazine is highly appealing but it remains a very difficult task because the degradation of hydrazine is thermodynamically more feasible than the cleavage of the N-H bond of NH3. As a result, any catalyst capable of activating NH3 will thus unavoidably decompose N2H4. Here we show that cavitation bubbles, created by ultrasonic irradiation of aqueous NH3 at a high frequency, act as micro-reactors to activate and convert NH3 to NH species, without assistance of any catalyst, yielding hydrazine at the bubble-liquid interface. The compartmentation of in situ produced hydrazine in the bulk solution, which is maintained close to 30°C, advantageously prevents its thermal degradation, a recurrent problem faced by previous technologies. This work also points towards a path to scavenge •OH radicals by adjusting the NH3 concentration.