  Temperature is a hallmark parameter influencing almost all magnetic resonance
properties (e.g., T\textsubscript{1}, T\textsubscript{2}, proton density,
diffusion and more). In the pre-clinical setting, temperature has a large
influence on animal physiology (e.g., respiration rate, heart rate, metabolism,
cellular stress, and more) and needs to be carefully regulated, especially when
the animal is under anesthesia and thermoregulation is disrupted. We present an
open-source heating and cooling system capable of stabilizing the temperature
of the animal. The system was designed using Peltier modules capable of heating
or cooling a circulating water bath with active temperature feedback. Feedback
was obtained using a commercial thermistor, placed in the animal rectum, and a
proportional{\text -}integral{\text -}derivative (PID) controller capable of
locking the temperature. Operation was demonstrated in a phantom as well as
mouse and rat animal models, where the standard deviation of the temperature of
the animal upon convergence was less than a tenth of a degree. An application
where brain temperature of a mouse was modulated was demonstrated using an
invasive optical probe and non-invasive magnetic resonance spectroscopic
thermometry measurements.
