  The resistive measurements were made to study the magnetic field-induced
antiferromagnetic (AF) - weak ferromagnetic (WF) transition in La$_2$CuO$_4$
single-crystal. The magnetic field (DC or pulsed) was applied normally to the
CuO$_2$ layers. The transition manifested itself in a drastic decrease of the
resistance in critical fields of ~5-7 T. The study is the first to display the
effect of the AF -WF transition on the conductivity of the La$_2$CuO$_4$
single-crystal in the parallel - to - CuO$_2$ layers direction. The results
provide support for the 3-dimensional nature of the hopping conduction of this
layered oxide.
