The differentiation of the bulbar region and arterial outflow tract of the developing mouse heart was investigated by analysing serial transverse sections through the heart region of mouse embryos isolated between the eleventh and fifteenth day of gestation. Over this period of time, we observed the configurational and cellular changes occurring within the wall of this region of the heart, being particularly interested in the histological appearance of the cellular constituents of the spiral ridges and their eventual apposition and fusion to form the spiral septum. We observed that the mesenchyme cells of which the ridges are largely composed are initially orientated in the direction of the outflow tract, but subsequently realign themselves when the individual ridges become oblique and spiral in their configuration. The tissue that gives rise to the spiral septum, namely the 'bulbar cushions' proximally, and the 'truncal cushions' in the rest of the outflow tract appear at all stages to be continuous structures. We saw no evidence that they initially develop as separate entities, and subsequently fuse. Furthermore, no evidence of cell death was observed in either the mesenchyme tissue or in the wall of the outflow tract. We have therefore suggested that, contrary to the findings of Pexieder (1978), pre-programmed cell death probably plays no significant part in the development of the spiral septum in the mouse, though we cannot exclude the possibility that there may be species differences between the events associated with spiral septum formation in avian and mammalian embryos. We conclude from our histological observations that the changes that occur in the arterial outflow tract in the mouse are probably brought about by the haemodynamic effect of the forces of blood flow impinging on its walls and that this initiates a series of events that are controlled to a considerable degree by pre-programmed genetic instruction.