ABSTRACT.- The objective of the study was to determine if a feeding system with a variable supply of grass promoted rapid changes in the fatty acid profile and technological and health indices of milk obtained from North American (NAHF) and New Zealand (NZHF) Holstein-Friesian cows. Two feeding strategies were conducted:fixed grass (GFix) and maximized grass intake when available (GMax). The results showed that as the grass intake increased in the GMax treatments, the relative amount of palmitic acid in milk decreased, whereas oleic, linoleic, linolenic, and conjugated linoleic acids increased, causing a reduction in the atherogenic, thrombogenic, and spreadability calculated indices. The changes occurred in rapid response to the changing diet, with reductions ranging from approximately 5 to 15% in the healthy and technological indices within a period of 15 d of grass intake increase. Differences were found between the 2 genotypes, with NZHF responding faster to changes in grass intake. © 2023, The Authors. Published by Elsevier Inc. and Fass Inc. on behalf of the American Dairy Science Association.