Primary cultures of dorsal root ganglia (DRG) sensory neurons can be protected against subsequent severe thermal or ischaemic stress by prior exposure to a mild thermal or ischaemic insult. The degree of protection correlates with the amount of 70 kDa heat shock protein (hsp70) induced by the mild stress. We show directly that over-expression of hsp70 alone is sufficient to protect DRG neurons against thermal or ischaemic stress with a given level of hsp70 over-expression providing greater protection against thermal stress. In contrast over-expression of the 90 kDa heat shock protein (hsp90) has little or no protective effect against either stress. These results are discussed in terms of the role of individual hsps in protecting neuronal cells against different stresses.