Observational data imply that Gigahertz Peaked and Compact Steep Spectrum (GPS and CSS) sources are the most likely candidates for being the precursors of the large scale classical double radio galaxies. They therefore provide a possibility of studying the birth and early evolution of radio sources, since little is known of the mechanisms triggering the radio activity. It has been suggested that mergers may play a role in the transport of gas into the nucleus and thus be involved in the onset of radio activity. If this is the case, unusual nuclear kinematics and a dense nuclear environment may be expected in the youngest AGN. I will present results from our HI absorption survey of GPS/CSS sources, and briefly discuss the plausibility of probing molecular circumnuclear gas in those objects.