The photocycle of bacteriorhodopsin (BR) was studied in the 0.3 microsecond to 10 s time interval after excitation, using a wide range of actinic light intensities (10 ns half-duration, 0.06-60 mJ/cm2), at neutral and alkaline pH values. The relative weights of the rapidly and the slowly decaying components of the M intermediate (Mf and M(s), respectively) and the yield of the third millisecond component, N(R,P), are the function of the exciting light intensity (density), while their lifetimes are not. The relative weight of M(s) is found to be a linear function of the portion of the BR molecules undergoing the photocycle. This suggests the existence of a cooperative interaction of the BR molecules arranged in the crystalline purple membrane sheets. Another source of M(s) is also found, which results a nonvanishing relative weight of M(s) even at very weak actinic light density values. The explanation for this may be a branching, or the heterogeneity of BR itself or with its environment. It is shown that the relative weights of the rising and decaying components of the M form(s) do not correlate directly with each other.