We derive Sobolev--Poincaré inequalities that
estimate the $L^q(d\mu)$ norm of a function on
a metric ball when $\mu$ is an arbitrary Borel
measure. The estimate is in terms of the
$L^1(d\nu)$ norm on the ball of a vector field
gradient of the function, where $d\nu/dx$ is a
power of a fractional maximal function of $\mu$.
We show that the estimates are sharp in several
senses, and we derive isoperimetric inequalities
as corollaries. 1991 Mathematics Subject Classification:
46E35, 42B25.