Commencing on day 100 ± 10 of lactation, 10 Italian Friesian cows averaging 32·6 kg milk per day were injected biweekly with recombinant bovine somatotropin (BST), while 10 other cows averaging 33-4 kg milk per day were injected with a placebo. Cows were offered ad libitum a total mixed diet throughout the lactation. The experiment lasted 16 weeks; milk samples were taken before and 16, 62 and 91 days after the commencement of BST injection. The objective was to determine the impact of biweekly injections of BST on the synthesis and proportion of milk fatty acids, particularly on cis and trans isomers and on fatty acids with an odd number of carbon atoms (C15 and C17). A decrease in the proportion of shortchain (C4 to C14) fatty acids (P < 0·001) and an increase in the proportion of long-chain (C16 to C20) fatty acids (P < 0·001) were observed in the first period of the treatment. With regard to cis and trans isomers, a considerable increase of the cis isomers (P < 0·001), particularly C18.1cis9 (oleic acid) was observed, while differences in trans isomers were not statistically significant. Concentrations of C15 (pentadecanoic acid) were not affected but heptadecanoic acid (C17) concentration showed a considerable increase (P < 0·01) but only in the sample taken after 16 days.