  We present a study of the intraday variability behaviour of a complete sample
of radio-selected BL Lac objects taken from the 1 Jy catalogue. In 28 out of 34
BL Lac objects (82\%) we detected intraday variability. 7 objects were observed
during several campaigns. None of them changed its variability behaviour. The
duty cycle in radio-loud BL Lac objects is very high, i.e. at least 0.8.
  The typical peak-to-peak amplitudes of the variability were 30\%. By means of
structure function and autocorrelation analyses we investigated the typical
time-scales of the variability and determined the activity of the BL Lac
objects. In 21 of the variable BL Lac objects we were able to measure a typical
time-scale, which lies in the range between 0.5 and 3 days.
  In general, the results can be explained by the standard model, where shocks
are propagating down a relativistic jet. However, in the two most densely
sampled observations we found variability characteristics, which cannot easily
be explained by the standard model alone, demonstrating that at least in these
objects alternative models should be taken into account. Nevertheless, all
models, which are able to explain variability on time-scales of days must be
able to consider the high duty cycle of these BL Lac objects.
