Immediate early genes are crucial intermediates in a cascade linking membrane stimulation to long-term alterations of neuronal activity. In the present experiment, we performed immunohistochemistry for c-Fos to determine the effects of persistent pain on cells of the hippocampus of male and female rats. Animals were subcutaneously injected with formalin (50 microl, 10%) and perfused: 2 h later, time 2; 24 h later, time 24; 24 h later after 20 min of the open-field test, time 24/OF. Controls were left undisturbed. In control, c-Fos was higher in females than in males in all hippocampal fields. In males at time 2, formalin increased c-Fos in the dentate gyrus (DG) and CA3 fields; at time 24, c-Fos returned to the control level; at time 24/OF, c-Fos was higher than in control in the DG, but not in the other fields. In the formalin-treated females at time 2 and at time 24, c-Fos levels were lower, or tended to be lower, than in control in all hippocampal fields; at time 24/OF, c-Fos levels in the DG were higher than in control and in males. In conclusion, persistent pain had different effects on c-Fos in the hippocampal subfields, depending on the time after treatment and the sex of the subject.