The objective the present study was to determine the influence of a supplemental methionine analogue, the isopropyl ester of 2-hydroxy-4-(methylthio)-butanoic acid, commercially available as MetaSmartTM, on lactation performance, particularly milk protein production. The effects of this preparation were compared with those of a rumen-protected form of methionine, marketed as SmartamineTM M. Experiments were conducted according to a 3 × 3 Latin square design and included 30 high-yielding dairy cows (22 Holstein and 8 Czech Fleckvieh) randomly allocated to three balanced groups. Cows were fed a basal diet based on maize silage, lucerne silage, lucerne hay, fresh brewer’s grains, and a concentrate mixture in the form of a total mixed ration ad libitum. The diet M was supplemented with MetaSmartTM (42.5 g/day) and diet S was supplemented with SmartamineTM M (19 g/day), while control diet C contained solvent-extracted soybean meal, which was added to achieve required levels of dietary protein. Each period lasted four weeks in total, including three preliminary weeks and one experimental week during which samples of milk and tail vein blood were taken. Supplementation of MetaSmartTM decreased dry matter intake of cows (18.96 kg) in contrast to the diet containing SmartamineTM M, for which dry matter intake was the highest (20.48 kg; P < 0.001). Despite decreased dry matter intake, the highest average milk yields were recorded for cows supplemented with MetaSmartTM (31.34 kg), which produced by approximately 1.14 kg (P < 0.001) and 0.78 kg (P < 0.01) more milk than cows fed diets C and S, respectively. As expressed by greater ratios milk/DMI, FCM/DMI, and ECM/DMI, the feed efficiency was improved in cows supplemented with MetaSmartTM.Both MetaSmartTM and SmartamineTM M dietary supplementation increased milk yield, milk protein concentrations, and yields and increased the prevalence of β-casein fraction in milk protein.