1. On a plane, $n$ circles intersect each other pairwise, the maximum number of intersection points is
$\qquad$.
- 1. $n(n-1)$.

When two circles intersect, they have at most 2 intersection points;
When three circles intersect, they have at most $2+4=6$ intersection points;
When four circles intersect, they have at most $2+4+6=12$ 
intersection points;
When $n$ circles intersect, they have at most
$$
2+4+\cdots+2(n-1)=n(n-1)
$$
intersection points.