The shapes of fluorescence induction curves in spinach chloroplasts, measured using double-flash pump-probe techniques, are shown to depend on the duration of the actinic flashes. For flash durations tau(0) </= 2 mus, the variable fluorescence F(nu) grows exponentially (or nearly so) with increasing fluence J of the actinic pulses and the fluorescence induction ratio R = F(max)/F(0) is </=2.6. When tau(o) >/= 50 mus, the shapes of the F(nu) vs. J curves are sigmoidal, and R > 3.2. Overall, the experimentally observed trends suggest that, as the duration tau(0) of the actinic pulses is increased, the degree of sigmoidicity, the deduced values of the interunit excitation transfer parameter p, and the fluorescence induction ratios R, also tend to increase. These results can be accounted for in terms of a simple double-photon hit model in which a dark lag time tau(1) = 0.4-10 mus between the two hits is necessary for the observance of sigmoidal fluorescence induction curves and relatively high R ratios. It is shown that, in principle, such a model can account for the exponential and sigmoidal shapes of the fluorescence induction curves either within the context of a lake model of the photosynthetic antenna bed (free transfer of excitation between photosynthetic units) or the isolated (puddle) model of photosystem II reaction centers. However, from the known values of the R ratio measured with actinic pulses of different durations, or under continuous illumination, the lake model offers a better description of the experimental phenomena than the puddle model.