1. The acid-soluble ribonucleotides of cow's milk and human milk were measured at different stages of lactation.2. Cow's milk contained mainly orotic acid with moderate amounts of CDP-choline, 5′-CMP, 3′,5′-cycic AMP. Cow's colostrum contained more than thirteen nucleotides (UDPH, orotic acid, 5′-CMP, 5′-UMP, UDPGA, GDP, GDPF, GDPM, 3′, 5′-cyclic AMP, 5′-AMP, 5′-GMP, UDP, CDP-choline).Both human milk and colostrum contained 5′-CMP, 5,′UMP, UDPAH, 5′-AMP, UDPGA, GDPM, 5 UDPH, 3′, 5′-cycic AMP, 3'-UMP, orotic acid, GDP, UDP and CDP choline.3. Several novel nucleotides of the UDP sugar type were detected in human milk and colostrum.4. The physiological and nutritional significance of these nucleotides was also discussed.