Of Rats, Mice, and Men

There are a number of reasons why we use mice and rats in laboratory testing. Mice and rats breed fast, which allows for the testing of generations in a few months, whereas it would take years to test at that level in humans. Along the lines of breeding, it is also worthy to note that the test subjects are inbred, so any genetic anomalies caused by the product over generations would be easier to track. When looking at product safety, or the potential threat posed by the product, we can look at various generations to find a modicum of proof, it is this level of testing that is required by overseers such as the Food and Drug Administration (FDA)(1).
When we wonder where we get the mice and rats for testing, we may think of a person out there catching them. The truth could not be further away from this. The test subjects must all be the same breed, and must have pure genetic histories. In addition to this they must undergo tests for genetic defects, because defects of this nature could affect the results of the experiment. In other words, the mice and rats used must have a pure genetic history before even being considered for testing.
Our little furry friends are small, which makes storage and maintenance easy. Large numbers of them can be tested in a relatively small space. They are also plentiful and cheap. You can get a large number of them for a minimal expenditure.
All of these factors go into the decision to use mice and rats, but the first factor to look at, is that we are mammals, and testing would do not be worth anything if not tested on a mammal. Mice and rats bear a close resemblance to humans genetically, biologically, and behaviorally. Coupled with the fact that these factors are well known of mice and rats, and we can more easily define the effects of the test product.
Owing to these similarities, we can create many in mice and rats many symptoms that are bound to conditions normally found in humans. In the past 20 years, we have become an even closer genetic match. Scientists have figured out how to breed "transgenic mice". These mice are genetically bred to host genes that closely resemble the genes that cause diseases in humans. In this manner, the results of the tests more closely approximate the results of a human test subject.
Along with transgenic mice, we also have what are referred to as "knockout mice". These knockout mice are genetically engineered with certain genes shut off. The mice with these inactive genes are very useful in evaluating the results of cancer causing chemicals. They are also useful in the assessment of the safety of certain drugs.
Some mice are used for normal and malignant human tissue research. These mice are known as SCID mice, because they have severe combined immune deficiency, which is to say that they were born naturally without an immune system. This makes them extremely useful in human tissue research(2).
In all, there are a great many reasons that we use rats and mice as test subjects, but in the end, it is because they are so very much like us, that the results are easily translated to human beings.

(1)http://www.wisegeek.org/why-are-rats-used-in-animal-testing.htm
(2) http://www.livescience.com/32860-why-do-medical-researchers-use-mice.html