Summary. The concentration of total carnitine (i.e. carnitine plus acetylcarnitine) was measured in seminal plasma and spermatozoa of men and rams. In ram semen, there was a close correlation between the concentration of spermatozoa and that of total carnitine in the seminal plasma, indicating that the epididymal secretion was the sole source of seminal carnitine. The percentage of total carnitine present as acetylcarnitine was 40% in seminal plasma and 70–80% in spermatozoa. The acetylation state of carnitine in seminal plasma was apparently not influenced by the metabolic activity of spermatozoa in ejaculated ram semen as no change was found in the plasma concentration of carnitine or acetylcarnitine up to 45 min after ejaculation. In spermatozoa, the activity of carnitine acetyltransferase (EC 188.8.131.52) was approximately equivalent to that of carnitine palmitoyltransferase (EC 184.108.40.206); and the activity of these enzymes was similar in ram and human spermatozoa but greater in rat spermatozoa. It is concluded that there is no correlation between the content of either total carnitine or the carnitine acyltransferases and the respiratory capacity of spermatozoa.