Plasma progesterone and LH profiles in dairy heifers following oestrus synchronization with PGF_2α and GnRH. Oestrus synchronization was accomplished in six Friesland heifers by two injections of PGF_2α, 11 days apart. During the ensuing synchronized oestrus, three of the six heifers received 2,5 ml GnRH solution 1M concomitant with the first of two inseminations (AI approximately 10 and 24 hours after oestrus observation). Hormone analyses of blood collected at 8-hourly intervals showed that the administration of GnRH exerted no effect on LH release, since the natural pre-ovulatory release had previously occurred.