After synchronization of oestrus, normally cyclic heifers (n = 31) received 2500 iu pregnant mares' serum gonadotrophin (PMSG) i.m. and had a progesterone-releasing intravaginal device (PRID) without the oestradiol capsule inserted on day 10 of the oestrous cycle and received 15 mg prostaglandin(PG) i.m. 48 h later. PRIDs were removed 96 h after insertion and 16 heifers received 1.0 mg GnRH i.m. while the controls (n = 15) received 10 ml saline i.m. All heifers were injected with anti-PMSG i.v. 10 later. Peripheral blood concentrations of PMSG, progesterone, oestradiol and LH were compared. Ovaries were collected on death 7 days after the GnRH or saline injection and the number of corpora lutea counted. Heifers were considered to have responded well (> 60 pmol l-1) or poorly (< 60 pmol l-1) to superovulation on the basis of the oestradiol concentration 24 h after PG administration. During PRID treatment, LH concentrations remained at basal values. In the heifers treated with GnRH, a single LH surge occurred 2.3 +/- 0.1 h (SD) after the GnRH injection with a maximum concentration of 14.6 +/- 2.3 (SEM) micrograms l-1 and a duration of 6-8 h. In 12 of the 15 control heifers, LH concentrations remained low (range 0.10-1.94 micrograms l-1) during the 72 h following the saline injection; three controls showed a spontaneous LH surge at 18, 23 and 23 h after the saline injection, respectively, with a maximum concentration of 6.0-12.5 micrograms l-1 and a duration of 10-12 h.(ABSTRACT TRUNCATED AT 250 WORDS)