Greening etioplasts in leaf tips of seedlings treated with varying, sublethal concentrations of metribuzin or its ethylthio analog for 12, 24, or 72 hr were examined by transmission electron microscopy to determine differential effects of these closely related herbicides in wheat ( Triticum aestivum L. “Daws”) and downy brome ( Bromus tectorum L.). In general, herbicide effects within a species were similar, but varied in terms of concentration required to induce the changes. In wheat, a loss of plastid invagination and overall plasticity of shape was followed, with increasing herbicide concentrations, first by a reduction in stroma lamellae and then by swelling and separation of grana thylakoids. Effects were similar in downy brome, with the distinction that at no time or concentration was any thylakoid swelling observed. After 72 hr, nontreated downy brome chloroplasts had accumuated starch, and all herbicide concentrations tested, except the 5 μ M ethylthio analog, inhibited these deposits completely. In both species, the ethylthio analog generally altered chloroplast ultrastructure earlier and more dramatically than did metribuzin, especially at concentrations of ≤50 μ M. In wheat, herbicide influence first became discernible at 24 hr, at ≥250 μ M metribuzin and at ≥50 μ M ethylthio analog. At 72 hr, damage was slight at 20, 50, and 250 μ M metribuzin, and severe at 20 and 50 μ M ethylthio analog. Downy brome chloroplast envelopes appeared wrinkled after treatment with both herbicides, but remained intact. No macroscopically discernible symptoms of tissue breakdown were evident in either species.