The temporal relationships of serum prolactin, oestrogen and LH concentrations during the perioestrous period were compared in prepubertal gilts induced to ovulate by PMSG and hCG and in mature gilts. In Exp. 1, 2 sustained prolactin surges, beginning 4 days and 1 day before the preovulatory LH surge, occurred in all mature gilts. A single preovulatory prolactin surge occurred in 3 prepubertal gilts, starting just before the preovulatory LH surge, but 4 prepubertal gilts had neither a prolactin

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... nor an LH surge. A status (prepubertal or mature) versus time interaction (P < 0\m=.\01) was detected for serum prolactin concentrations. A preovulatory oestrogen surge occurred in all gilts but was of lesser magnitude (P < 0\m=.\01) and duration (P < 0\m=.\05) in the prepubertal gilts without prolactin and LH surges compared to mature gilts and of lesser magnitude (P < 0\m=.\01)compared to prepubertal gilts with prolactin and LH surges. The relative timing of the oestrogen surge in prepubertal gilts corresponded with that of mature gilts when adjusted to the LH surge (if present) but was delayed (P < 0\m=.\01) in all prepubertal gilts if standardized to the hCG injection. In Exp. 2, mature gilts were examined to determine whether 2 perioestrous prolactin surges were characteristic of all cycling gilts. Of 9 gilts, 8 exhibited an initial prolactin surge 4\p=n-\5 days before oestrus and 5/9 gilts exhibited a periovulatory prolactin surge. The presence of 2 perioestrous serum prolactin surges was not a requirement for subsequent pregnancy maintenance. The temporal relationships amongst serum oestrogen, prolactin and LH concentrations during the perioestrous period were dramatically different in prepubertal gilts induced to ovulate and mature gilts. Such differences may contribute to early pregnancy failure in the prepubertal gilt.