The need for centimetric accuracies set by the application of the VLBI technique to geodesy implies a considerable computational effort, because of the intrinsic complexity of the model and because there is an increasingly large number of calibrations and corrections which can be accounted for only via software. The program VLBI 3 (Robertson, 1975) has been developed for the geodetic and astrometric analysis of VLBI data. It includes an accurate theoretical model of the observables and is supported by a number of routines for parameter fitting and input/output operations with data and results. The original VAX version of VLBI 3, due to N. Bartel and M.I. Ratner, runs in batch mode and requires routines which are in general unavailable in standard VAX systems. We have prepared (Caporali and Sylos Labini, 1982) a modified VAX version of VLBI 3. This version runs on our standard VAX/VMS computers and contains a number of changes in the FORTRAN source which allow to the user a real time interaction with the program. In addition, having a Tektronix graphic station at our disposal, we decided to replace the existing plot package - which used the line printer - with a “ad hoc” graphic program which permits interactive display of the results of each run of VLBI 3. Our work was mostly concerned with the input/output sections of VLBI 3. The theorical model of the VLBI observables has, for the moment, been left unchanged. We have, however, noticed that the theorical model could be updated and made more precise, e.g. in the computation of the nutation terms and of the aberration. This updating and a more extensive geodetic and astrometric analysis of VLBI data are planned to be done next.