Long slit Halley comet spectra taken at ESO Observatory after the perihelion have been analyzed. Relative intensity radial profiles along and perpendicular to the sun direction have been obtained for CN (Δv=0), C2 (Δv=0) and C3 (4050 A) bands. Interpretation of these profiles with the vectorial model allowed us to trace the variability of the gas production rate vs. time before the observations. It has been shown that the comet inereased its gas production rate of a factor 3 on 22.9 March 1986, then it decreased to half the 23.1 March 1986.