A perifusion system of anterior pituitary (AP) tissue was used to investigate the temporal interaction of growth hormone-releasing factor (GRF) and somatostatin (SRIF) in the control of GH secretion in two pig breeds, Göttingen Miniature Pig (GMP), a small obese breed, and German Landrace (GLR), a conventional lean breed. AP tissue pieces derived from sexually mature ovariectomized animals were perifused (6 replicates per treatment) and fractions were collected at 10 min intervals. Basal GH release (ng.ml-1.mg-1 AP) in GLR was twice that of GMP (P < 0.001). Exposure to 10 min pulses of 1 nM GRF repeated 3 times at 2 h intervals resulted in rapid stimulatory GH responses (area under the curve) which became attenuated (P < 0.05) over time in GMP but not in GLR. Surprisingly, during and following the exposure of AP tissue from GMP to 10-, 20-, or 40-min pulses of 10 nM SRIF alone, GH release was markedly stimulated (P < 0.05), while AP tissue from GLR only showed a weak rebound GH release after SRIF pulses. With AP tissue from GLR low concentrations (0.1 nM SRIF) amplified GRF-induced GH release, whereas 1 nM or 10 nM SRIF inhibited GRF-induced GH release. However, concomitant exposure of AP tissue from GMP to 0.1, 1 or 10 nM SRIF during a GRF pulse markedly enhanced the GH response (P < 0.05), compared to 1 nM GRF alone, except for 1 nM SRIF which inhibited the GH response to the first GRF pulse. Thus the presence of SRIF, and not only its withdrawal, is an important factor in setting the timing and duration of GH pulses in both breeds. In GLR the concentration of SRIF is more important than the duration and/or type of SRIF pulse. In contrast, in GMP type and/or duration of SRIF pulses seem to be crucial to optimize pulsatile GH release and even determine peak height of GH pulses caused by GRF. These findings indicate clear breed differences in the role of SRIF and in the control of GH release by the interplay of GRF and SRIF. The "paradoxical' effect of SRIF suggests that the role of SRIF is much more complex than that of a mere inhibitor and whose real role could be a modulator either of GH pulse and/or GRF action on GH release.