One of the factors which determine the fraction of incident light back-scattered by a tissue is its blood content. Oestrogenic hormones and drugs increase blood content in the female back-scattered from a circumferential section of the vaginal wall. The probe function and response were validated in physiological tests, and in experiments with ewes treated with oestradiol. Features particular to our design are immunity from ambient light levels and from source and sensor drifts with time or temperature, short response time, and user convenience. These features are important for long-term application with objective of determining whether the pre-ovulatory rise in oestrogen levels in women can be correlated with a change in vaginal back-scattering.