The effects of 28 days of heavy resistance training while ingesting the alleged testosterone-boosting supplement, NMDA, were determined on body composition, muscle strength, serum cortisol, prolactin, and hormones associated with the hypothalamo-pituitary- gonadal (HPG) axis. Twenty resistance-trained males engaged in 28 days of resistance training 4 times/wk while orally ingesting daily either 1.78 g of placebo (PLAC) or NMDA. Data were analyzed with separate 2 x 2 ANOVA (p < 0.05). Criterion measures involved body composition, muscle strength, serum cortisol, prolactin, and gonadal hormone levels [free and total testosterone, luteininzing hormome (LH), gonadotrophin releasing hormone (GnRH), estradiol], and were assessed before (Day 0) and after (Day 29) resistance training and supplementation. No changes were noted for total body water and fat mass in response to resistance training (p > 0.05) or supplementation (p > 0.05). In regard to total body mass and fat-free mass, however, each was significantly increased in both groups in response to resistance training (p < 0.05), but were not affected by supplementation (p > 0.05). In both groups, lower-body muscle strength was significantly increased in response to resistance training (p < 0.05); however, supplementation had no effect (p > 0.05). All serum hormones (total and free testosterone, LH, GnRH, estradiol, cortisol, prolactin) were unaffected by resistance training (p > 0.05) or supplementation (p > 0.05). The gonadal hormones and cortisol and prolactin were unaffected by 28 days of NMDA supplementation and not associated with the observed increases in muscle strength and mass. At the dose provided, NMDA had no effect on HPG axis activity or ergogenic effects in skeletal muscle.

• In response to 28 days of heavy resistance training and NMDA supplementation, similar increases in muscle mass and strength in both groups occurred; however, the increases were not different between supplement groups.

• The supplementation of NMDA had no preferential effect on augmenting testosterone or decreasing estrogen, cortisol, and prolactin.

• While resistance training was effective in increasing muscle mass and strength, it was not preferentially due to NMDA supplementation.

• At the dose provided, NMDA supplementation for 28 days combined with resistance training does not increases muscle mass and strength due to its ability to elevate endogenous testosterone levels and lower cortisol and prolactin when compared to placebo.