The US government does have a rather nasty history of experimentation on citizens but it is not a matter of kidnapping. Usually unethical experimentation has involved a lack of informed consent — you have people who have agreed to take part in something but they don’t really know what they’ve gotten themselves into, and the experimenters have deliberately not told them.
So a few of the most famous of these:
* Radioactivity experiments. The US government was very interested in how radioactive elements, notably plutonium, moved through human systems. In some cases this involved researchers injecting terminally-ill patients with plutonium to see how it was excreted through their urine and feces, how it accumulated in their bodies. There were even cases of several dozen mentally disabled students at the Fernald State School being given radioisotopes, without their or their parents’ consent. There isn’t much evidence that any of this experimentation did real harm — the doses were very small — but it definitely violated the Nuremberg Code (informed consent, and most of these experiments were not about finding positive benefits). For more on this, see Eileen Welsome, _The Plutonium Files_.
* Tuskegee syphilis study. Perhaps the most famous case of unethical experimentation in the USA. The US Public Health Service conducted a multi-decade study of _untreated_ syphilis, which they did by giving placebos to African-American patients who thought they were getting treated. They started in 1932, when good treatments were lacking, but kept the study going until 1972, decades after which good treatments were available. [More details here](https://www.cdc.gov/tuskegee/timeline.htm).
* MK-ULTRA and other CIA-funded psychological/pharmaceutical experiments. The CIA had several Cold War programs dedicated to finding ways to manipulate human minds. Some of these involved research into what would today be considered torture techniques, others involved funding operations and medications to alter personalities and minds. Famously these involved lots of experimentation with LSD, including the using of it on people without their consent, and were inadvertently responsible for some part of the rise of the counter-culture in the 1960s (Ken Kesey was one of the LSD-study participants, and later became an evangelist of the drug). One of the most horrific things I have read: a woman went in for treatment for mood swings and mild depression from a CIA-funded doctor, and left feeling cured. But she also left without any memories of her three children. Yeah. This and more in Rebecca Lemov’s, _World as Laboratory_, chapter 10.
There are other cases of unethical experimentation — testing on prisoners and conscientious objectors, for example, that would no longer be seen as satisfying Nuremberg Code requirements — but the above three are what people tend to think of first when it comes to this. The backlash against these kinds of stories is why things like Institutional Review Boards were created (for better or ill — there are many who think IRBs go so far in the other direction that they make doing human studies prohibitively difficult). (I have not included [compulsory sterilization programs](http://alexwellerstein.com/publications/wellerstein_statesofeugenics.pdf) in this list, because it was regarded as a kind of treatment/punishment, not research, per se.)
I don’t know of any cases where kidnapping was involved. Again, typically the subjects have been large “captive” populations in jails, asylums, conscientious objectors, hospitals, clinics, etc., so kidnapping would seem unnecessary.