Herbicides have been used to control tall larkspur to prevent cattle deaths. There is some evidence that tall larkspur may become more toxic and more palatable during the desiccation process following herbicide treatment. A series of studies were conducted to measure toxic alkaloid concentration in tall larkspur following treatment with metsulfuron, picloram, and glyphosate. Herbicides were applied to larkspur plants grown in a greenhouse (1990), to field plots infested with larkspur (1993), and to individual larkspur plants in the field (1995). Leaves were harvested up to 14 or 28 d following treatment, and toxic alkaloids were measured. Metsulfuron increased toxic alkaloid concentration in the 1990 greenhouse study and in 1993 field plot studies, and increased both concentration and absolute amount of toxic alkaloids in 1995 individual plant treatments. Alkaloids in plants treated with picloram either remained unchanged or declined slightly in all three studies and were similar to control plants. In the 1995 study, glyphosate caused the relative concentration of alkaloids to increase as the plants desiccated, but the absolute amount of alkaloids was similar to control plants. Picloram and glyphosate did not reduce alkaloid concentration; thus, the risk of poisoning remains until plants desiccate. Metsulfuron increased alkaloid concentration, thus increasing the risk of poisoning. Cattle should not be allowed to graze sprayed areas until larkspur desiccates and withers.