The spontaneous activity of 454 single hypothalamic neurons was recorded in 42 chronically ovariectomized rats after severance of all neural connections with the diencephalon. In 15 of these diencephalic island preparations progesterone was administered immediately before the recording session (and just after deafferentation of the diencephalon) and oestrogen 72 h beforehand. Thirteen rats were given two injections of oestrogen at these times and the remaining 14 rats were similarly treated with equal volumes of oil. Blood samples were obtained from all rats just before each hormone or oil injection, and 4, 5, 6 and 7 h after the second one, for subsequent measurement of plasma luteinizing hormone (LH) concentration. Only the group of rats given progesterone at the time of the second injection showed a significant increase in plasma LH concentration during the recording period. There was however some individual variation and the greatest LH surge was obtained from a rat given two injections of oestrogen. For steroid-treated rats the size of the LH surge was significantly correlated (P < 0.01) with the mean firing rate of the neurons recorded in the preoptic and anterior hypothalamic areas (p.o.-a.h.). No similar correlation could be established for p.o.-a.h. cells recorded in oil-treated rats or for cells recorded in other parts of the hypothalamus in steroid-treated rats. The mean firing rate of all p.o.-a.h. cells recorded from rats treated with oestrogen followed by progesterone was significantly higher (P < 0.05) than in either of the other two groups of animals. The oestrogen-progesterone treatment also significantly changed the regularity of discharge of the slow firing (< 2 Hz) p.o.-a.h. cells, but this phenomenon could not be related to any alteration in plasma LH concentration. The experiments have demonstrated for the first time that the magnitude of the steroid-stimulated LH surge in ovariectomized rats is significantly correlated with the increase in the electrical activity of p.o.-a.h. neurons.