Fans of the furry little houseguest known as the guinea pig may want to shut their eyes or sing very loudly during certain parts of this discussion, but it's all about the human thirst for knowledge!

Animals used for laboratory tests are not selected randomly from local pet stores or breeders, but are shipped in directly from specialized companies which raise genetically pure strains. This purity eliminates many of the problems scientists face when observing the results of their experiments, since all of the animals should be in similar health beforehand. For decades, one of the test animals of choice was the guinea pig, considered to be heartier and easier to observe than mice or rats. To be used as a guinea pig, therefore, meant to be the test subject for medical or scientific experiments.

Eventually, however, the guinea pig became less and less popular as a test animal for laboratory experimentation. Compared to other rodents, a guinea pig has a long gestational period, which means it could take months before generations of guinea pigs could be studied for genetic damage. A guinea pig is also highly sensitive to drugs such as penicillin, which does not make it an ideal candidate for medical research. Many laboratories have already switched to using mice for their research, and there are only two strains of laboratory-grade guinea pigs now available for research purposes. In 2006, a major guinea pig breeding facility in Great Britain bowed to public pressure and agreed to stop production.

In the common vernacular, being used as a guinea pig suggests becoming a tester for an unproven process or product. If a friend recently graduates from a beauty school and wants to perform her first professional haircut, for example, the brave friend in the chair is being used as a guinea pig. A cook experimenting with a new recipe may also seek out a volunteer to act as a guinea pig. Others may decide to observe the guinea pig's reactions before committing themselves to a similar taste test. Unlike the actual rodent, a human guinea pig is often aware of the dangers involved and agrees to take the risk.

Sometimes a large company will use one of its smaller divisions as a guinea pig in order to gauge public interest in a new product line or service. By observing both the positive and negative results of the experiment, the company can make a rational decision on whether or not to invest more money and time into the project. Being used as a guinea pig in a rhetorical sense can be both rewarding and risky at the same time. There is an element of sacrifice inherent with becoming a guinea pig, but there is also a sense of accomplishment if the experiment leads to new advances.