We have employed density functional theory in statistical mechanics to study the nucleation of the gas-liquid transition inside a slit pore. This is the simplest kind of pore, in which a fluid is confined between two infinite and identical planar surfaces. Equilibrium capillary condensation in such systems has been studied in the past. Here, we explore the kinetics of nucleation of the new phase and show that the critical nucleus can either be attached to one of the planes or can bridge the two planes, depending on the conditions of the experiment. We show that the macroscopic capillarity approximation is not quantitatively accurate, but can provide a useful qualitative picture provided that the line tension is incorporated. Comparisons are made with recent simulations of phase transitions in slit pores. (C) 2001 American Institute of Physics.