Three naturally occurring loline derivatives (N-formylloline, N-acetylloline, and N-methylloline) and 15 new N-acylloline derivatives were synthesized from loline (hexahydro-N-methyl-2,4-methano-4H-furo[3,2-6]pyrrol-3-amine). These compounds were all tested for phytotoxic activity. Several concentrations of each were applied exogenously to alfalfa (Medicago sativa L.) and to annual ryegrass (Lolium multiflorum L.) seeds to determine dose-response relationships. Of the endogenous compounds only N-formylloline demonstrated significant phytotoxicity by inhibiting germination of annual ryegrass. Eight of the synthetic N-acyl derivatives of loline were phytotoxic and exhibited 50% reductions in germination and seedling length with doses of less than 1.5 X 10-7 mol/seed. N-Dodecanoyl- and N-(p-rc-hexylbenzoyl)loline were the most phytotoxic derivatives tested and exhibited ID50 values of 5 X 10-8 mol/seed for the reduction of annual ryegrass seedling length. The N-dodecanoyl and N-tetradecanoyl derivatives were the most active; activity decreased as the chain length of the acyl groups became either longer or shorter. The loline derivatives were only slightly selective as exhibited by general 2-fold greater toxicity against annual ryegrass over alfalfa. © 1990, American Chemical Society. All rights reserved.