We investigate martingale inequalities
in noncommutative $L^p$-spaces associated with a von Neumann
algebra equipped with a faithful normal state. We prove the
noncommutative analogue of the classical Burkholder inequality
on the conditioned (or little) square function and extend the
noncommutative Burkholder--Gundy inequalities from Comm. Math. Phys.
189 (1997) 667--698 to this
nontracial setting. We include several related results.