Microvariability observations of two TeV emitting blazars have been made fr
om the Canary Islands International Observatories since 1990. We present a
small sample of typical results from the study. We find that, in contrast t
o the long-term light curve, Mark 501 is more active than Mark 421, suggest
ing that these, superficially similar objects, are really quite different i
n characteristics. (C) 1999 Elsevier Science B.V. All rights reserved.