The present work involved the administration of both ethynyl estradiol and levonorgestrel to groups of rats, followed by determination of the homocysteine excretion rate in urine. The results indicate that a statistically significant difference exists between the excreted levels of homocysteine in the urine of both control and levonorgestrel-treated rats and the levels shown by rats treated with ethynyl estradiol. The implications of these results are discussed, especially with respect to observations which indicate that homocysteine may be a precipitating factor in the development of thrombosis. Also included in this paper is a study which confirms the identity of the HPLC peak as being homocysteine by forming a radioactive derivative of this particular sulphydryl-containing amino acid, and then analysing the resulting mixture by TLC.