To investigate changes in the proliferative activity of bone marrow cells in canine cyclic hematopoiesis, nonadherent cells were incubated for 1 h with tritiated thymidine either immediately after the cultures were established or following an 18-h preincubation period. The data suggest that changes in thymidine incorporation show a 12- to 14-day cycle that consists of two distinct phases. During the first six days of the cycle (from peripheral neutropenia to relative neutrophilia), two peaks of incorporation were observed. During the second phase (corresponding to the neutrophilia and oncoming neutropenia), thymidine incorporation was uniformly lower than control values. The change from an apparently cyclical process to a low stable value occurred after the wave of marrow myelopoiesis and close to a time point (days 8-10 of the cycle) at which we have recently suggested significant changes in cell release and/or proliferation take place. The data can be interpreted in the context of a periphery-to-stem-cell feedback loop through an intermediate cell population, probably of myeloid precursors.