Abstract: Apparatus for detecting and indicating the commencement of inhalation by a patient under test includes a housing through which a gas flow due to the inhalation is directed. A light source and a light receptor positioned adjacent to transparent portions of the housing define a light path between them which transversely intersects the gas flow, the receptor being adapted to provide an output signal in response to incident light from the source. A movably positioned vane mounted inside the housing and normally biased to a position substantially clear of the light path, moves to a position progressively blocking the light path upon commencement of gas flow due to inhalation. Means responsive to a change in the receptor output signal resulting from blockage of the light path provides indication of the flow due to the inhalation commencement.