The acid-soluble ribonucleotides of cow's, goat's and sheep's milks were measured by enzymic and ion-exchange chromatographic procedures at different stages of lactation. Colostra and milk from ruminant species contained orotic acid and 13 well-identified nucleotides: AMP, CMP, GMP, UMP, UDP, GDP, UDP-glucose, UDP-galactose, UDP-N-acetyl-glucosamine, UDP-N-acetyl-galactosamine, UDP-glucuronate, GDP-mannose and GDP-fucose. Cow's, goat's and sheep's colostrum contained significant amounts of nucleotides, which increased in amount from the moment of parturition, to reach a maximum 24–48 h later. The nucleotide concentration decreased thereafter with advancing lactation. Cow's milk contained substantial amounts of orotic acid, which increased during lactation, whereas in goat's and sheep's milk no increase took place. The cytidine and adenosine derivatives in ruminants' milk did not change significantly during lactation. The UDP-glucose/UDP-galactose ratio was measured in ruminants' milk at different stages of lactation.