Nicotinamide adenine dinucleotide (NAD+) is an essential cofactor in many cellular processes and its role in the production of energy is well characterised. Nicotinamide mononucleotide (NMN) is a precursor for NAD+ biosynthesis and NAD+ elevating treatments have been found to improve oocyte quality in cattle, mice and pigs [1-3], suggesting that NAD+ is vital during oocyte maturation. The aim of this study was to examine the effects of inhibiting nicotinamide mononucleotide adenylyltransferase (NMNAT), a key enzyme in both the Preiss-Handler and salvage pathways of NAD+ biosynthesis, on the maturation of porcine oocytes in vitro. Porcine oocytes from small antral follicles were matured for 44 h in defined maturation medium either without (control) or with gallotannin (55 µM; NMNAT inhibitor), NMN (100 µM) or gallotannin and NMN combined. At 44 h, maturation rates were determined, and mature oocytes were fixed and stained to assess spindle assembly and chromosome segregation. Inhibition of NMNAT reduced the proportion of oocytes that reached MII (25.59 ± 1.13 vs. 84.67 ± 0.03 and 93.89 ± 3.09%; p < 0.0001), reduced spindle length (0.38 ± 0.38 vs. 6.56 ± 0.29 and 6.72 ± 0.45 µm; p < 0.001) and decreased spindle width (0.90 ± 0.90 vs. 9.14 ± 0.38 and 7.86 ± 0.50 µm; p < 0.0001) compared with control and NMN supplemented oocytes. Co-supplementation of gallotannin with NMN rescued spindle width to a diameter similar to that of control oocytes (7.36 ± 3.16 µm; p > 0.05) but had no effect on maturation rates or spindle length. The results show that inhibition of NMNAT by gallotannin dramatically impaired oocyte meiotic progression and spindle assembly. The finding that co-supplementation with NMN partially restored spindle assembly indicates that NAD+ production via the salvage pathway is involved in this crucial process.