Systems and methods are described for managing initialization of virtual machine instances within an on-demand code execution environment or other distributed code execution environment. Such environments utilize pre-initialized virtual machine instances to enable execution of user-specified code in a rapid manner, without delays typically caused by initialization of the virtual machine instances. However, because the number of pre-initialized virtual machine instances maintained at an on-demand code execution environment is typically limited, insufficient number of pre-initialized virtual machine instances may be available at the on-demand code execution environment during times of heavy use. Embodiments described herein utilize pre-trigger notifications to indicate to the on-demand code execution environment that subsequent requests to execute user-specified code are likely to occur. The on-demand code execution environment may therefore pre-initialize additional virtual machine instances in preparation for the subsequent requests, reducing delay that would be required to initialize the instances after obtaining to the requests.