Poisonous plants and noxious weeds are often chemically examined to de
termine concentrations of secondary metabolites which are responsible
for their toxic or biological activity. This study examined sample siz
e requirements and sample methods necessary to quantify accurately the
concentrations of individual and total toxic alkaloids in two tall la
rkspur populations. A high performance liquid chromatography analytica
l method was utilized to determine toxic alkaloid concentrations in al
l leaves from three individual plant stems and leaves from the remaini
ng stems (remainder) from each of 50 plants in each population, To obt
ain high precision in quantifying toxic alkaloids in the larkspur popu
lations (within 2.5 to 5% of the population mean, 0.95 confidence), ve
ry large numbers of samples (>50-200) were required. However, lower pr
ecision (within 10% of the population mean, 0.90-0.95 confidence) requ
ired only 20 samples. Similarly, testing parameters relating to toxin
concentration in tall larkspur populations within 5 or 10% of the popu
lation mean also required hundreds of samples at power levels of 0.95
and alpha-levels of 0.05. Relaxing power and a-level requirements to 0
.80 and 0.1 respectively, reduced sample size to about 30. The means o
btained by four different sampling methods were similar (P > 0.05). Al
kaloid concentrations in leaf samples from single stems were highly co
rrelated to whole-plant leaf (remainder) samples (r(2) greater than or
equal to 0.76), indicating that harvesting leaves from single stems p
rovided representative samples of the entire plant. The results indica
te the difficulty in obtaining accurate information about toxins in po
isonous plant populations for risk assessment by livestock producers o
r extension agents and demonstrate the necessity for efficient analyti
cal methodology. Researchers evaluating concentrations of plant compou
nds in other weeds or toxic plants should consider variability, sampli
ng procedure, and sample size before experiments begin.