ABSTRAKPenelitian ini bertujuan untuk mengkaji mekanisme sintesis mentholpada berbagai periode pencahayaan tanaman M. piperita L. Penelitiandilakukan di Instalasi Penelitian Balai Penelitian Tanaman Obat danAromatik, Lembang, Jawa Barat, dari bulan Januari 2000 hingga Juli 2000.Penelitian dilakukan tiga tahap. Tahap pertama membuat variasilingkungan cahaya, tahap kedua penyulingan dan analisis komponenminyak dengan kromatografi gas spektrometer massa dan tahap ketigamerunut lintasan biosintesis menthol. Penelitian menggunakan rancanganacak kelompok 5 perlakuan, yaitu L 0 (panjang hari normal sebagaikontrol), L 1 (pemutusan periode gelap 1 jam, pukul 21.00-22.00 mulaiumur 30 hari), L 2  (pemutusan periode gelap 1 jam, pukul 21.00-22.00mulai umur 60 hari), L 3 (penambahan cahaya 4 jam, pukul 18.00-22.00mulai umur 30 hari), dan L 4 (penambahan cahaya 4 jam, pukul 18.00-22.00 mulai umur 60 hari). Hasil penelitian menunjukkan bahwaperubahan lingkungan mempengaruhi mekanisme sintesis menthol didalam tanaman Mentha piperita L. Perubahan lingkungan mengubahlintasan menthol yang selanjutnya mengubah komponen minyak dan mutumenthol. Tingginya kadar menthol dan rendahnya menthofuran padapenambahan cahaya 4 jam terjadi melalui penghambatan pembentukansenyawa menthofuran dengan mereduksi pulegon menjadi menthol,sehingga menthol meningkat, sedangkan pada perlakuan kontrol terjadioksidasi pulegon ke menthofuran sehingga menthol rendah. Penambahancahaya 4 jam mulai umur 30 hari setelah tanam menghasilkan minyakdengan kadar menthol paling tinggi yaitu 54,89% dan menthofuran palingrendah yaitu 7,83%.Kata kunci :  Mentha, Mentha piperita L., periode pencahayaan, hasil,komposisi minyak, Jawa BaratABSTRACTThe effect of photoperiod on menthol sysnthesis ofMentha piperita L.Research on the effect of photoperiod on menthol synthesis ofMentha piperita L, was carried out at the Experimental Garden of InstituteReseach for Medicinal and Aromatic Crops, Lembang, West Java, fromJanuary until July, 2000. The objective is to study the machanism ofmenthol synthesis in relation with the manipulation of light periode, threesteps were taken: The first step was manipulation of environment using TLlamps (two experiments), the second step was distillation and analisis ofpeppermint oil from their products with gas chromatography and massspectrometry, and the third step was tracing the pathway on mentholbiosynthesis. At the first experiment, 5 treatments were given i.e. (1)control or normal light period, (2) four hours light supplement at the age of30 days and 60 days after planting, and (3) one hour interruption of darkperiod at the age of 30 days and 60 days. The result showed that the effectof light period manipulation can change the pathway of mentholbiosynthesis and oil component and finally the quality of menthol. Fourhours light supplement at the age of 30 days after planting could enhancethe menthol content and reduce menthofuran by blocking the reaction frompulegone to menthofuran, so the pulegone was reduced into menthon andmenthol. Four hours light supplement at 30 days after planting showed thehighest menthol content (54.89%) and the lowest menthofuran (7.83%).Control treatment (normal light period) showed the lowest mentholcontent, due to no reduction of pulegone into menthon, but pulegone wasoxidized into menthofuran. Without additional light the menthol contentdecreased and the menthofuran content increased.Key words : Mentha, Mentha piperita L., oil composition, photoperiod,yield, West Java