Absorption, translocation, and metabolism of 14C-glufosinate were studied in three annual and two perennial weed species. Young seedlings of Setaria faberi, Chenopodium album, Cassia obtusifolia, Solanum carolinense, and Asclepias syriaca were treated with foliar-applied 14C-glufosinate, and plant tissues were harvested 12, 48, and 72 h after treatment (HAT). Absorption of 14C-glufosinate was initially rapid, but increased only slightly after 12 h in all species. Glufosinate absorption was highest in S. carolinense (73% of applied radioactivity), followed by S. faberi (54%), C. obtusifolia (44%), C. album (41%), and A. syriaca (37%) 72 HAT. Translocation of radioactivity out of the treated leaf was species dependent and did not increase much with time in all weed species. S. carolinense and S. faberi translocated the highest amounts of absorbed radioactivity out of the treated leaf with 49 to 59% moving to the upper foliage. S. faberi translocated the highest amount of absorbed radioactivity to the roots (12 to 14%), while C. album translocated the least (2 to 3%). TLC analysis of plant extracts showed that 14C-glufosinate was not metabolized in S. faberi, C. obtusifolia, S. carolinense, and A. syriaca. A glufosinate metabolite with an Rf value matching that of methyl-phosphinico propionate was detected in C. album. Treatment with ammonium sulfate (AMS) increased glufosinate absorption in S. faberi and C. obtusifolia 12 HAT, but decreased absorption in C. album. Treatment with pelargonic acid (PA) did not affect glufosinate absorption in any of the species tested. Treatment with AMS and PA did not affect glufosinate translocation in any of the five weed species. Treatment with AMS and PA did not influence the metabolism of glufosinate in any of the five weed species studied. These results show that differential absorption and translocation seem to explain the greater sensitivity of the annual and perennial weeds to glufosinate. Treatment with ammonium sulfate may increase the efficacy of glufosinate in perennial weeds.