  We present a flux variability study of simultaneous RXTE and EUVE
observations of the highly variable Seyfert galaxy NGC4051. We find a strong
correlation between variability in the EUV and medium energy X-ray
bands,indicating that both are sampling the same power-law continuum. The lag
between the two bands is less than 20 ks and, depending on model assumptions,
may be <1 ks. We examine the consequences of such a small lag in the context of
simple Comptonisation models for the production of the power-law continuum. A
lag of <1 ks implies that the size of the Comptonising region is less than 20
Schwarzschild radii for a black hole of mass >1E6 solar masses.
