We consider the class of sequential estimation systems containing additive Gaussian noise channels. Let

be a random variable representing the source and

be stochastic processes representing the signal and the channel output, respectively. Denote by

an estimate of

based on observing

from 0 to

, where

is a random variable determined by a certain stopping rule of the decoder depending on a realization of

. Let

be the distortion of

relative to

and

be the rate distortion function of

with respect to the distortion measure

. Denote by

and

the available average signal power and the noise power level, respectively. We show that

and hence

.. That is, given the average distortion

, the average transmission time required can be no smaller than

. Conversely, given the average transmission time

, the average distortion can be no smaller than

.