Abstract: A sampler is invented for collecting human or animal gastrointestinal fluid samples. The sampler is about the size of a medicine pill for oral administration. It has a hollow body 10, an opening 20, a seal 30, and a vacuum interior 40. When the sampler enters the gastrointestinal tract of a patient, the seal is dissolved or disabled and the surrounding fluid is sucked into the sampler by the vacuum inside. The sampler is then recovered from the patient's stool and the gastrointestinal fluid and substances within the sampler can be collected and analyzed.