The study evaluated the impact of mGnRHa ([D-Tle6, Pro9, NEt]-mGnRH), with or without metoclopramide, on release of luteinizing hormone (LH) and sex steroids and on viability of ova in tench Tinca tinca. Four experimental groups were subjected to the following treatments given as intraperitoneal injection: 0.9% NaCl (C); 25 μg kg− 1 mGnRHa (Gn); 25 μg kg− 1 mGnRHa with 20 mg kg− 1 metoclopramide (GnM); and or 20 mg kg− 1 metoclopramide (M). GnM treatment induced LH surge reaching maximum concentrations 6 h post-treatment followed by a slight but significant decline (P < 0.05). Gn group showed gradually increasing LH values (P < 0.05). No significant differences were found between GnM and Gn treated groups in 17β-estradiol (E2) or testosterone blood level. Significant decrease from baseline (P < 0.05) was found in E2 values at the end of trial period in both Gn and GnM. Testosterone values increased with Gn and the GnM, peaking 12 h post-treatment and dropping to baseline levels at 24 h. GnM was associated with significant increase of 17α, 20β-dihydroxy-4-pregnen-3-one (17,20β-P) values at 6 h (P < 0.05), with its concentration peaking at 12 h similar to that in Gn. Both treatments induced ovulation in a high proportion of females, while M and C group showed no ovulation. No significant differences were found in ovulation rate, latency, ovum weight, or relative fecundity. Nevertheless, significant differences (P < 0.05) between Gn and GnM treatment were observed in fertility (77% vs. 54%) and hatching (40% vs. 23%). GnM stimulated rapid LH release with a potentiating effect on 17,20β-P secretion, possibly the source of the adverse effects on ovum viability. We describe for the first time the negative impact of metoclopramide combined with mGnRHa hormone treatment on ovum quality in tench.