The paper proposes a specification for the transient and steady-state responses of a hydroelectric power station operating in frequency-control mode. It can be used during design, testing and commissioning as well as forming the basis for contractual agreement on performance during normal operation. The specification gives a generic definition of how the electrical power should respond to step, ramp and random changes in frequency. The rationale for the proposed specification is discussed. A prototype transfer function is proposed as an aid to formulating a step response specification. The length of record required during random testing is discussed. The use of the specification is illustrated by considering the response of the Dinorwig pumped-storage hydroelectric station.