  Analysis of rapid variability at 4.85 GHz for the BL BLac object 0925+504 is
presented and discussed. The structure functions (SF) are investigated with
both refractive and weak interstellar scintillation (RISS/WISS) models
analytically. Parameters obtained with these models are quantitatively
compared, suggesting that the emission region of IDV is remarkably compact and
the responsible interstellar scintillation medium (ISM) lies very close to the
observer. Furthermore possible evidence of annual modulation of the variability
timescales is detected in this source. Our findings indicate that the observed
rapid variability in 0925+504 is predominantly caused by a scattering screen
located along the line of sight to the source, at a distance of $\sim 110\,pc$
to the observer.
