We have performed resistance measurements on thin films of the high-te
mperature superconductor YBa2Cu3O7 (YBCO) in applied magnetic fields t
o above 200 T (2 MegaOersted) at temperatures as low as 2.5 K. The fie
lds are produced by an explosively driven flux-compression system. We
can see a particularly clear onset, without replotting the data, of th
e ''hydrodynamic'' flow of vortices probably because of the very fast
increasing field. The low temperature ''critical field'' for the field
parallel to the c-axis of the sample is 135 T. Our data in the other
direction are still preliminary. We discuss possible interpretation of
our results.