In spores of the fern Dryopteris filix-mas, we have followed loss of Pf, activity shortly after sowing, using a null method: with a (start) pulse of red light a steady state level of Pf, is established 0.5 h after sowing, thus inducing germination. 7.5 -23.5 h thereafter, the Pf, level still remaining is tested with a second saturating pulse. If this (test) pulse is given with various wavelengths, it increases or decreases the germination response, and these changes are used to determine, by interpolation, the actual Pr, level at the time of the test pulse. Assuming that there is dark destruction or dark reversion underlying this apparent Pf, decay, its kinetics are calculated. For both these assumptions the kinetics are significantly slowed down as the age of the spores increases (i.e. the time of storage, after harvest, at 6 "C). Interestingly, with increasing spore age a single pulse of red light becomes increasingly more effective in the induction of germination. This correlates well with the slower decay, thus enabling a higher Pf, level to act for a longer time. Virtually identical results are obtained for D. paleacea.