In contrast to the impact of nutritional intervention on post-exercise muscle protein synthesis, little is known about the potential to modulate protein synthesis during exercise. This study investigates the impact of protein co-ingestion with carbohydrate on muscle protein synthesis during resistance type exercise. Ten healthy males were studied in the evening after consuming a standardized diet throughout the day. Subjects participated in 2 experiments, in which they ingested either carbohydrate or carbohydrate with protein during a 2h resistance exercise session. Subjects received a bolus of test drink prior to and every 15 min during exercise, providing 0.15 g·kg^-1·h^-1 carbohydrate with (CHO+PRO) or without (CHO) 0.15 g·kg^-1·h^-1 protein hydrolysate. Continuous intravenous infusions with L-[ring-¹³C₆]phenylalanine and L-[ring-²H₂] tyrosine were applied, and blood and muscle biopsies were collected to assess whole-body and muscle protein synthesis rates during exercise. Protein co-ingestion lowered whole-body protein breakdown rates by 8.4±3.6% (P=0.066), compared to the ingestion of carbohydrate only, and augmented protein oxidation and synthesis rates by 77±17 and 33±3%, respectively (P<0.01). As a consequence, whole-body net protein balance was negative in CHO, whereas a positive net balance was achieved following the CHO+PRO treatment (-4.4±0.3 vs 16.3±0.4 µmol phe·kg^-1·h^-1, respectively; P<0.01). In accordance, mixed muscle protein fractional synthetic rate (FSR) was 49±22% higher following protein co-ingestion (0.088±0.012 and 0.060±0.004 %·h^-1 in CHO+PRO vs CHO treatment, respectively; P<0.05). We conclude that, even in a fed state, protein co-ingestion stimulates whole-body and muscle protein synthesis rates during resistance type exercise.