A sampling device for obtaining samples of internal body substances in the digestive system is a swallowable capsule which allows a sample of the body substance to enter the capsule through an inlet which opens in the digestive tract following contact with the substance to be collected. The capsule inlet initially is sealed and, after the capsule has been swallowed, the inlet opens following contact with the body substance. An inner chamber preserves a vacuum or substantial under pressure when the inlet is sealed by a blocking member adjacent to the inlet. The blocking member is elastic and has a flow permitting configuration which admits the body substance into the inner chamber and a flow preventing configuration which blocks the inlet from the inside of the chamber when the pressure difference has been equalized.