A theory of the thermoelectric power due to the interlayer hopping assisted
by spin fluctuations has been developed. The prediction of the theory capt
ures the main features of experiment. Thus, we argue that the c-axis thermo
electric power exhibiting metallic behaviour while the c-axis electronic co
nductivity appears to be nonmetallic in the underdoped cuprate may be prope
rly understood within the theory.