The numerical density of nuclear pores was assessed on freeze-fracture replicas of hypothalamic arcuate neurons from adult male and female rats. In females the nuclear pore density fluctuated during the oestrous cycle and was higher in oestrus than in pro-oestrus, metoestrus and dioestrus. Nuclear pore density in males and in metoestrus and dioestrus females was similar. The nuclear pore density in male rats was significantly less than that in oestrus and pro-oestrus females. The variation of the number of pores per unit nuclear volume and the total number of pores per nucleus was similar to the variation of the numerical density of nuclear pores. These results provide morphological evidence of cyclic changes in neuronal nucleocytoplasmic traffic during the ovarian cycle.