  We analyze the role of bars in the build-up of central mass concentrations in
massive, disk galaxies. Our parent sample consists of 3757 face-on disk
galaxies with redshifts between 0.01 and 0.05, selected from the seventh Data
Release of the Sloan Digital Sky Survey. 1555 galaxies with bars are identified
using position angle and ellipticity profiles of the $i$-band light. We compare
the ratio of the specific star formation rate measured in the 1-3 kpc central
region of the galaxy to that measured for the whole galaxy. Galaxies with
strong bars have centrally enhanced star formation; the degree of enhancement
depends primarily on the ellipticity of the bar, and not on the size of the bar
or on the mass or structure of the host galaxy. The fraction of galaxies with
strong bars is highest at stellar masses greater than $3 \times 10^{10}
M_{\odot}$, stellar surface densities less than $3 \times 10^8 M_{\odot}$ and
concentration indices less than 2.5. In this region of parameter space,
galaxies with strong bars either have enhanced central star formation rates, or
star formation that is {\em suppressed} compared to the mean. This suggests
that bars may play a role in the eventual quenching of star formation in
galaxies. Only 50% of galaxies with strongly concentrated star formation have
strong bars, indicating that other processes such as galaxy interactions also
induce central star-bursts. We also find that the ratio of the size of the bar
to that of the disk depends mainly on the colour of the galaxy, suggesting that
the growth and destruction of bars are regulated by gas accretion, as suggested
by simulations.
