In an attempt to improve the sensitivity and selectivity of thin-layer chromatographic analysis for the detection of strychnine and crimidine in biological samples, a rapid high-performance thin-layer chromatographic (HPTLC) method with densitometry is described. Fortified dog serum and stomach content samples were analyzed after extraction with chloroform. Quantitation was achieved by densitometry in the ultraviolet (UV) range (260 nm) of HPTLC silica gel 60 plates. Detection of trace levels as low as 5 ng proved feasible. Linearity was obtained over a range of 10-250-ng deposits for crimidine and 12.5-250-ng deposits for strychnine with simple or F254 plates. No interferences were observed in the UV spectra (220-380 nm) when peaks obtained with HPTLC of the standard substances and positive biological contents were scanned.