A technique of activating nucleation sites by heating them with laser beams has been developed to enable direct measurements of interactions between active nucleation sites. Interactions between two, three and four simultaneously active nucleation sites in various arrangements on 25 mum thick copper and titanium foil in saturated water pool boiling were studied experimentally. The experiments were performed with heated spot diameters ranging from 1.66 to 5.23 mm and heat fluxes up to 560 kW/m(2). The interaction between two laser activated nucleation sites occurs as a net decrease of activity of both sites. It is possible that the activity of both decreases simultaneously, or the activity of one increases, while the activity of the other nucleation site decreases simultaneously. Similar behaviour was also observed in the experiments involving three and four active nucleation sites. In the extreme case, one nucleation site can deactivate the other. Local surface characteristics can play an important role in the interaction between the nucleation sites.