The changes in splenic germinal center (GC) cell proliferation were measured during primary and secondary responses to a T-dependent antigen in vivo to examine the regulation of the GC reaction. Adult C3H/HeN mice were immunized with sheep red blood cells and boosted 7 or 21 days later. GC cell proliferation was assessed by measurement of GC cell birth rates using a stathmokinetic technique. Actual GC growth and regression were assessed in terms of total splenic volume and number. Pre-existing GC had a mean cell birth rate of 33 cells/1000 cells/h. The GC reactions following each immunization showed a biphasic pattern of changes in cell birth rate, comprising an initial fall immediately succeeded by a transient, but significant, increase. These fluctuations occurred earlier in secondary compared to primary responses. Significant increases in total GC volumes succeeded the peaks of cell birth rate following both primary and early secondary immunization. However, there was a substantially smaller increase following later secondary immunization. We propose that the initial cell birth rate reduction is due to inhibition of pre-existing GC clones and represents one component of the phenomenon of GC dissociation. The succeeding peak birth rate represents early, massive proliferation of newly activated antigen-specific clones. The different patterns of GC expansion, despite similar proliferative responses, may reflect different pathways of differentiation dependent on the timing of antigenic stimulation.