The effect of prolonged hyperprolactinaemia on the secretion of LH, progesterone and oestradiol, and its relationship to the maintenance of pregnancy was examined in pigs. Twelve crossbred, pregnant gilts were injected i.m. with 1.5 mg haloperidol kg-1 body weight (n = 6) or vehicle (n = 6) once a day from day 60 to day 66 of pregnancy. Blood samples were collected at 08:00, 12:00, 16:00, 20:00, 24:00 h from day 60 to day 67 and every 15 min for 4 h (08:00-12:00 h) on days 60, 63 and 66. Plasma concentrations of prolactin were higher (P < 0.001) in haloperidol-treated gilts than in control gilts (121.3 +/- 4.3 ng ml-1 and 13.6 +/- 0.4 ng ml-1, respectively). Hyperprolactinaemia completely inhibited the pulsatile secretion of LH and diminished (P < 0.001) basal peripheral concentrations of LH (hyperprolactinaemia, 0.3 +/- 0.04 ng ml-1 and control, 0.6 +/- 0.005 ng ml-1). Despite the inhibition of LH release in hyperprolactinaemic gilts, plasma concentrations of progesterone were higher (P < 0.001) than in the control group (20.8 +/- 0.6 and 12.6 +/- 0.2 ng ml-1, respectively). Oestradiol concentrations were not different between groups, although oestradiol tended to be higher in hyperprolactinaemic gilts than in the control group throughout the sampling period (29.1 +/- 1.9 versus 23.7 +/- 1.6 pg ml-1, respectively). Abortion did not occur in any of the gilts. These results are the first to demonstrate that induced hyperprolactinaemia during the second half of pregnancy (days 60-66) will drastically suppress the major porcine luteotrophin but not affect pregnancy maintenance in pigs.(ABSTRACT TRUNCATED AT 250 WORDS)