Optogenetics and chemogenetics are neuromodulation techniques used to study neuronal pathways. Viral vectors containing optogenetic or chemogenetic probes require at least a week after injection to produce effective expression levels in neurons. Therefore, injections need to be done in neonatal mice to enable experiments that require mice younger than 30 days of age. Here, we describe a protocol for performing surgery on neonatal mice and brain injection of a viral vector. The procedure utilizes cryoanesthesia and a pressure injector with a micropipette that can be directly injected through the skull of neonatal mice. Compared to other approaches, this protocol is relatively easy to implement and takes only minutes to perform, additionally allowing for increased numbers of injections. In the examples shown in this chapter, viral vectors were successfully delivered into the auditory cortex and the hippocampus as indicated by labeled expression of the optogenetic/chemogenetic probes. This technique provides a method for investigators to perform surgical injections and optogenetic/chemogenetic experiments in neonatal mice.