Many environmental and occupational chemicals are known to affect the central and/or peripheral nervous system, causing changes that may result in neurological and psychiatric disorders. Because of the limited accessibility of the mammalian nervous tissue, new strategies are being developed to identify biochemical parameters of neuronal cell function, which can be measured in easily obtained tissues, such as blood cells, as potential markers of the chemically-induced alterations occurring in the nervous system. This review includes a comparative analysis of the effects of mercurials on calcium signalling in the neuroadrenergic PC12 cells and rat splenic T lymphocytes in an attempt to characterize this second messenger system as a potential indicator of subclinical toxicity. The suitability of neurotransmitter receptors in blood cells, such as the sigma binding sites, as biological markers of psychiatric disorders is also discussed.