SUMMARY: Plasma levels of luteinizing hormone (LH) were measured by radioimmunoassay in serial samples obtained from unanaesthetized, unrestrained rats by means of intracardiac cannulae (i) in relation to the critical period of neural facilitation (CP) during the afternoon of pro-oestrus and (ii) before and after an injection of sodium pentobarbitone given at specific times in relation to the critical period. Samples were also obtained by cardiac puncture before and after hypophysectomy carried out at specific times in relation to the CP. The effect of pentobarbitone or hypophysectomy on ovulation was also determined in the same animals. The CP as defined by pentobarbitone injection and hypophysectomy lasted from 15.00 to 16.00 h in the majority of rats. During this period the LH levels rose gradually above the basal value, and rose sharply thereafter to reach peak values at 17.00–18.00 h. Pentobarbitone given before the CP delayed the surge by 24 h, while its administration at the end of the CP caused an abrupt fall in plasma LH, indicating a reliance on the neurogenic stimulus both to initiate and to maintain the LH surge. LH levels reached immediately after pentobarbitone administration or hypophysectomy were correlated with the subsequent pattern of ovulation. Only a small increase in LH, amounting to about 14% of the peak values attained during the afternoon of prooestrus, was required for ovulation per se. This amount is similar to the small rise in circulating LH observed during the CP. The bulk of LH secreted during the 'pre-ovulatory surge' does not appear to be required for ovulation.