The contractile vacuole pores (CVPs) in Tetrahymena pyriformis are usually found to the left of ciliary meridians (CVP meridians). When the CVP meridians are experimentally rotated 180 degrees ('inverted'), the CVPs are now found to the right of the rotated CVP meridians. In other words, the CVP is almost invariably found on the same side with respect to the CVP meridian. Thus, the 2 sides of the CVP meridian have different morphogenetic properties and such differences are determinative in the asymmetrical fine-positioning of the CVP. The number of CVPs per animal and their general placement on the animal (i.e. with which ciliary meridians the CVPs are associated) are known to be a function of the total number of ciliary meridians possessed by the animal. Rotation of the CVP meridians affects both the number of CVPs per animal and their general placement. The specificity of such effects appears to depend on whether both, or one, or which one, of the CVP meridians is rotated. Rotation of ciliary meridians may be used as a tool in analysing the mechanism determining the number of CVPs per animal and their general placement.