A new method of quantifying the dynamics of the human dark adaptation function is presented. This method is based on nonlinear regression analysis and allows the derivation of a number of clinically useful indices of dark adaptation ability. The analysis method is applied to literature data describing the effect of ageing on dark adaptation and to clinical data from a retinitis pigmentosa patient. Ageing was seen to decrease the rate of adaptation in the cone portion of the dark adaptation function, and to have little effect on the rate of adaptation in the rod portion of the dark adaptation function. One case of retinitis pigmentosa was associated with increased rod and cone thresholds but little change in the rate of adaptation of the cone portion of the dark adaptation function. It is suggested that this analysis method may offer advantages in quantifying changes in dark adaptation dynamics.