Abstract: One embodiment of my invention comprises a tool string for testing a wellbore formation that includes a production inlet, an injection outlet, and a sampler apparatus. Fluid is taken from a production zone, into the tool string through the production inlet, out of the tool string through the injection outlet, and into the injection zone. Within the interior of the tool string, the sampler apparatus takes samples of the fluid flowing therethrough. In another embodiment, a large volume of sample fluid is trapped within the interior of the tool string, such as between two valves, and is removed from the wellbore along with the tool string subsequent to the test. In another embodiment, the tool string includes at least one perforating gun to perforate one of the production and injection zones. The tool string may also include two perforating guns to perforate both the production and injection zones.