The Srivani believed that experimentation on other species was an acceptable method of gaining medical knowledge, which they claimed might cure physical and psychological disorders that affected millions.
Their normal method of experimentation involved using a phase variance of 0.15 to render themselves, their ships and their devices both invisible and intangible. They then attached to the hull of a passing starship and spent several weeks carrying out experiments on the crew. These experiments ranged from changes in body chemistry to complex DNA manipulation carried out by genetic tags, attached to the host DNA base pairs. These tags utilized a level sub-molecular technology well beyond anything developed by Starfleet. They did not usually go out of their way to kill a test subject, but considered minimal fatalities as well as deformaties acceptable.
In 2374, for a period of at least several weeks a group of at least fifty-six Srivani used the USS Voyager as a testing ground. The experiments included adjusting the DNA of Commander Chakotay to make him prematurely age and stimulating the dormant Mylean DNA in Neelix, causing him to express the DNA handed down from his Mylean great-grandfather. They also forced Captain Janeway to become increasingly erratic by increasing her dopamine levels.
They were discovered by Seven of Nine and The Doctor who revealed them to the rest of the crew. Since their technology could not be defeated, Janeway flew Voyager at a nearby binary pulsar, almost destroying the ship. The Srivani, realizing that they could be killed, retreated, losing one ship in the process. (VOY: "Scientific Method")