Abstract The absorption of metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4 H)-one] and its ethylthio analog (ethyl-metribuzin) [4-amino-6-(1,1-dimethylethyl)-3-(ethylthio)-1,2,4-triazin-5(4 H)-one] by protoplasts of downy brome ( Bromus tectorum L.) and jointed goatgrass ( Aegilips cylindrica Host) was assayed by a modified silicone oil centrifugation method. In these experiments, 75 to 83% of the protoplasts were intact after centrifugation. Herbicide uptake was maximum within 30 sec, the shortest measurement period. The absorption of both herbicides was greater than predicted based solely on the volume of protoplasts, indicating that the herbicide had partitioned and/or bound to the membranes. The absorption of ethyl-metribuzin and metribuzin by protoplasts was similar and did not vary with temperature or species. Herbicide absorption by protoplasts of both species was linear with increasing concentrations (0.01 to 100 μ M) and did not approach saturation. In thylakoids, oxygen evolution (photosynthetic electron transport) was inhibited more by metribuzin than by ethyl-metribuzin, and inhibition correlated with increases in temperature. Thus, the increase in triazinone activity in cells exposed to increasing temperatures appears to be due to an increased inhibition of oxygen evolution in thylakoids and is not related to increased herbicide binding.