6. Let $P$ be the midpoint of the height $VH$ of the regular quadrilateral pyramid $V-ABCD$. If the distance from point $P$ to the side face is 3 and the distance from $P$ to the base is 5, then the volume of the regular quadrilateral pyramid is $\qquad$ .
6. 750 .

As shown in Figure 2, construct $P F \perp$ plane $V B C$ at point $F$.
According to the problem, we know $V P=5, V H=10$.
Then $V F=\sqrt{V P^{2}-P F^{2}}=\sqrt{5^{2}-3^{2}}=4$.
Since points $P, H, M, F$ are concyclic,
$$
\begin{array}{l}
\Rightarrow V P \cdot V H=V F \cdot V M \Rightarrow V M=\frac{25}{2} \\
\Rightarrow H M=\sqrt{V M^{2}-V H^{2}}=\frac{15}{2} \Rightarrow A B=15 . \\
\text { Therefore, } V_{\text {regular tetrahedron }}=\frac{1}{3} V H \cdot A B^{2}=750 .
\end{array}
$$