This study tested the proposition that Na K - ATPase activity could be involved in the morphogenetic aspects of mouse blastocyst formation by facilitating the localization of certain organelles to apposed cell borders, the production of nascent blastocoele fluid, and cavitation. It was assumed that such Na K - ATPase activity would be sensitive to varying concentrations of external K ( K o)—which would alter plasma membrane potentials—and to ouabain—which would directly alter Na K - ATPase function. Morulae were cultured for 40 hr in varying concentrations of K o and/or ouabain and were observed for their ability to form blastocoeles (cavitate) and to localize mitochondria to apposed cell borders. Cavitation was accelerated when K o was decreased from the control value of 6.0 to 0.6 m M and was delayed when K o was increased to 25 m M. With K o at 6.0 m M, 10 −5 M ouabain accelerated cavitation while 10 −4 M ouabain delayed cavitation and reduced the total number of embryos that cavitated by the end of the 40-hr culture period. With K o at 0.6 m M, 10 −5 M ouabain now delayed cavitation while 10 −4 M ouabain almost completely inhibited it. When K o was increased to 25 m M, 10 −5 M ouabain again accelerated cavitation while 10 −4 M ouabain delayed—rather than inhibited—cavitation. Morphometric analyses at the electron microscopic level showed changes in the distances of mitochondria from apposed cell borders with conditions that accelerated or delayed cavitation and these changes differed for inside and outside cells of the morula. These observations are consistent with the proposition that Na K - ATPase activity could be involved in the localization of organelles to apposed cell borders, the production of nascent blastocoele fluid, and in cavitation during mouse blastocyst development.