The influence of several parameters on the kinetics of activation of the progesterone receptor in the cytosol of rabbit uterus is described. The estimation of the proportion of activated receptor is based on the differential affinity of the activated and non-activated forms of the receptor for phosphocellulose. Under appropriate conditions binding to phosphocellulose can be used as a test of activation and gives results similar to those obtained with DNA--cellulose, or isolated cell nuclei. The kinetics of receptor activation is temperature-dependent and compatible with a first-order reaction at all temperatures tested. The thermodynamic activation energy of this reaction is 67.8 kcal mol-1. The progesterone receptor can be activated to various extents by increased ionic strength or by dilution of the cytosol with buffers of low ionic strength, and in all cases the activation follows apparent first order kinetics. At a concentration of 0.4 M NaCl, 70--80% of the receptor can be converted into the activated form. The activated and non-activated forms of the receptor appear to be in equilibrium. Salt-activated and heat-activated receptor can be transformed to a non-activated form by decreasing either the salt concentration, or the temperature of incubation. The rate of dissociation of the steroid from the activated form of the receptor is indistinguishable from that observed with the non-activated form, but the activated receptor is more thermolabile. Upon centrifugation on sucrose gradients there are no major differences in the sedimentation behaviour of the two forms of the receptor.