  We study dynamics of the measurement process in quantum dot systems, where a
particular state out of coherent superposition is observed. The ballistic
point-contact placed near one of the dots is taken as a noninvasive detector.
We demonstrate that the measurement process is fully described by the
Bloch-type equations applied to the whole system. These equations clearly
reproduce the collapse of the density-matrix into the statistical mixture in
the course of the measurement process. The corresponding dephasing width is
uniquely defined. We show that the continuous observation of one of the states
in a coherent superposition may accelerate decay from this state -- in
contradiction with rapidly repeated observations, which slow down the
transitions between quantum states (the quantum Zeno effect).
