The role of physical environmental conditions on uptake, translocation, and degradation of the32P-labelled systemic phosphonate,O-methylO-p-methylthiophenyl methylphosphonothionate, on cotton plants was investigated. Uptake, translocation, and degradation were measured by using radiochemical and bioassay techniques.A lengthening of the photoperiod from 10 to 14 hr increased rate and amount of radioactivity taken up and translocated by the cotton plants, improved insecticidal action of the systemic, and increased the degradation rate of the systemic. A further lengthening to 24 hr reversed the trends. Therefore, a point is reached at which lengthening of the photoperiod does not increase uptake and translocation of a systemic because too long a photoperiod upsets the plant's physiological functions.High humidity slowed the rate of uptake of the systemic and, in some way, reduced the chloroform-soluble fraction markedly at the 24 hr sample. At the 3-day sample, the chloroform-soluble fraction increased to about the same percentage of radioactivity as was found in the low humidity experiments, i.e., 17 and 30% higher than the 1 day sample at 65° and 80°F, respectively. High humidity slowed the rate of degradation of the chloroform-soluble fraction; the difference was greater between high and low humidity at 65° than at 80°F. For both humidities, the rate of degradation was much slower at 65° than at 80°F.Increase in temperature increased uptake and translocation of the systemic and increased the rate of degradation.