In prelim, expts. with the culture of guayule (Parthenium argentatum) plants, only little rubber was accumulated by this plant under greenhouse conditions. Alteration of various factors such as water supply, amt. of available N and day length in no case induced extensive rubber formation comparable to that found in field grown plants. Low night temps. did, however, favor rapid and extensive rubber formation, 40-45[degree]F appearing to be optimal or near optimal. Night temps. of 50[degree]F were less effective and temps. of 60-80[degree]F ineffective. The combination of low day temp. with low night temp. appeared to be less favorable for rubber formation than high (80[degree]F) day temp. combined with low night temp. Although responses in growth and rubber formation to low night temp. were evident within 1 month after transfer of plants to the low night temp. condition, still the response in rubber formation became progressively more striking after 2 and 4 months. Plants in their 2d yr. of growth responded more strikingly than plants only a few months old. The data suggest that the rapid rubber accumulation noted in guayule plants under field conditions during the fall and winter may be primarily a response to the low night temps.