Can you respond to your own spell on the stack by casting another spell?

When you cast a spell, it is placed on the stack and then players receive priority in turn order (CR 117.3b). As the active player, you receive priority first after your spell is placed on the stack. This means you can immediately cast another spell or activate an ability in response to your own spell before it resolves.

Priority must be passed by all players in succession without anyone taking an action before the stack resolves (CR 117.4). So as long as you have a legal way to cast another spell — the mana, a valid target if required, and the timing is legal — you can respond to your own spell.

This is a fundamental part of stack-based gameplay. Spells and abilities resolve one at a time, last-in-first-out (LIFO), so the most recently cast spell resolves first (CR 608.1).

Example: You cast Lightning Bolt targeting your opponent. Before it resolves, you cast a second Lightning Bolt also targeting your opponent. The second Bolt resolves first dealing 3 damage, then the first Bolt resolves dealing another 3 damage — for 6 total damage from the two spells.

There is one key condition: the spell you want to cast in response must be castable at that moment. Sorcery-speed spells generally cannot be cast this way (CR 307.1), but instant-speed spells and abilities with flash can be cast while your own spell is on the stack.