Gas chromatography-coupled electrophysiological recordings (GC-EL) from olfactory sensilla within the capsule of Haller's organ of the tick Amblyomma variegatum indicate the presence of a number of stimulants in rabbit and bovine odours, and in steer skin wash. Some of these stimulants were fully identified by gas chromatography-mass spectrometry analysis and by matching electrophysiological activity of synthetic analogues as: 1) hexanal, 2-heptenal, nonanal, furfural, benzaldehyde, and 2-hydroxybenzaldehyde (in all extracts); 2) heptanal, 2-, 3-, and 4-methylbenzaldehyde, and gamma-valerolactone (only in bovine and rabbit odour). Careful examination of the electrophysiological responses permit characterization of 6 receptor types: 1) a benzaldehyde receptor, 2) a 2-hydroxybenzaldehyde receptor, 3) three types of receptors responding differently to aliphatic aldehydes, and 4) a lactone receptor.