  A significant fraction of planetary nebulae (PNe) exhibit collimated
outflows, distinct narrow kinematical components with notable velocity shifts
with respect to the main nebular shells typically associated with
low-ionization compact knots and linear or precessing jet-like features. We
present here a spatio-kinematical investigation of a sample of twelve PNe with
morphologies in emission lines of low-ionization species suggestive of
collimated outflows. Using archival narrow-band images and our own
high-dispersion long-slit echelle spectra, we confirm the presence of
collimated outflows in Hen 2-429, J 320, M 1-66, M 2-40, M 3-1, and NGC 6210
and possibly in NGC 6741, for which the spatio-kinematical data can also be
interpreted as a pair of bipolar lobes. The presence of collimated outflows is
rejected in Hen 2-47, Hen 2-115, M 1-26, and M 1-37, but their morphology and
kinematics are indicative of the action of supersonic outflows that have not
been able to pierce through the nebular envelope. In this sense, M 1-66 appears
to have experienced a similar interaction between the outflow and nebular
envelope, but, as opposed to these four PNe, the outflow has been able to break
through the nebular envelope. It is suggested that the PNe without collimated
outflows in our sample are younger or descend from lower mass progenitors than
those that exhibit unambiguous collimated outflows.
