  We report the results of a study of the Hall effect and magnetoresistance in
single crystals of Sr2RuO4 in which Sr^(2+) has been substituted by La^(3+)
(Sr(2-y)La(y)RuO(4)) or Ru^(4+) by Ti^(4+) (Sr(2)Ru(1-x)Ti(x)O(4)). For undoped
Sr2RuO4, the purity is so high that the strong-field Hall coefficient can be
measured for fields above 4 T. The conventional weak-field Hall coefficient as
a function of doping shows a sharp jump and sign change at y ~ 0.01 that is
unrelated to either a sharp change in Fermi-surface topography or a magnetic
instability. The implications of these results are discussed.
