Aims: Melatonin is the most powerful antioxidant and protects sperm, oocyte, and embryo against oxidative stress. The effect of tryptophan, which is the building block of melatonin, on follicular melatonin levels and IVF outcomes is unknown. The objective of this study was to investigate the effect of tryptophan administration, a precursor for melatonin, on the levels of intrafollicular melatonin with the aim to reveal the correlation between tryptophan and the total number and quality of oocytes as well as clinical pregnancy rates. In addition, we aimed to examine the effect of melatonin increased by tryptophan on uterine and ovarian blood flow. Material and Method: Out of 103 patients who applied to Ondokuz Mayıs University Hospital, IVF clinic for IVF treatment, 51 patients were administered a 100 mg dose of tryptophan orally (Group A) and 50 control patients who were randomly selected did not receive tryptophan (Group B). Firstly, follicular melatonin levels were compared between Group A receiving tryptophan and Group B without tryptophan. Both groups were also compared according to the oocyte count, oocyte count, fertilized oocyte count, embryo count, and pregnancy rates, ultimately. In addition, all patients were measured for uterine and ovarian artery blood flow by vaginal ultrasound on the day of OPU. Results: There were no differences in age (32.16±3.82 years vs 33.06±4.44 years) (p=0.276), BMI (28.45±2.82 kg/m)2vs 28.15±3.03kg/ m2 (p=0.602) and peak estradiol levels (2451.69±469.75 pg/ml vs 2420.26±443.71 pg/ml) (p=0.73) between the groups. Group A exhibited high levels of melatonin in the follicular fluid with a mean value of 259.8 pg/ml, whereas Group B had 91.3 pg/ml (p<0.001). There were found significant differences in the oocyte count (9.08±3.22 vs 7.66±1.89) (p=0,008), mature oocyte count (7.2±2.8 vs 6.1±1.8) (p=0,021) and fertilized oocyte count (6.35±2.44 vs 5.28±1.69) between group A and group B. Pregnancy rates were higher in group A (35.3%). The pregnancy rate (30%) was lower in Group B, which did not receive tryptophan and had low melatonin levels in follicular fluid. However, there was no statistically significant difference. Uterine, ovarian artery systolic and diastolic blood flows of Group A were significantly lower than Group B (p<0.001). Conclusions: Administration of tryptophan to IVF patients significantly increases the level of melatonin in follicular fluid. The results demonstrate that high levels of melatonin in follicular fluid may increase oocyte count and quality although they do not significantly improve clinical pregnancy rates.