  Our investigation of the magnetotransport in two charge ordered manganites
with similar magnetic ground states reveals that the origin of
magnetoresistance can not be concluded from the isofield resistivity, $\rho
$(T, constant H), measurements alone. Both
Pr$_{0.5}$Sr$_{0.41}$Ca$_{0.09}$MnO$_{3}$ (PrSrCa) and
La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ (LaCa) show a ferromagnetic transition (T$_{C}$ =
260 K for PrSrCa, 230 K for LaCa) followed by an antiferromagnetic transition
(T$_{N}$ = 170 K for PrSrCa, 140 K for LaCa). These compounds show
qualitatively similar magnetotransport : Below the irreversibility temperature
T$_{IR}$, field cooled (FC) resistivity is lower than zero field cooled (ZFC)
and decreases continuously with T, whereas the ZFC $\rho $(T, H) resembles $\
$the behavior of $\rho $(T, H = 0 T). The value of $\rho $(ZFC)/$\rho $(FC) is
$\approx $ 10$^{4}$ at 5 K and $\mu_{0}$H = 7 T in both compounds. However,
isothermal magnetic measurements suggest distinct origins of magnetoresistance
: Field cooling enhances ferromagnetic phase fraction in LaCa whereas it drives
PrSrCa into a metastable state with high magnetization. The distinct origins of
magnetotransport is also reflected in other magnetic history dependent
properties.
