We compared transgenic Drosophila larvae varying in hsp70 copy number to assess the consequences of Hsp70 overexpression for growth and development after heat shock. Exposure to a mildly elevated temperature (36 degrees C) induced expression of Hsp70 (and presumably other heat shock proteins) and improved tolerance of more severe heat stress, 38.5-39.5 degrees C. We examined this pattern in two independently derived pairs of extra-copy and excision strains that differed primarily in hsp70 copy number (with 22 and 10 copies, respectively). Extra-copy larvae produced more Hsp70 in response to high temperature than did excision larvae, but surpassed the excision strain in survival only immediately after thermal stress. Excision larvae survived to adulthood at higher proportions than did extra-copy larvae and grew more rapidly after thermal stress. Furthermore, multiple pretreatment reduced survival of 1st-instar extra-copy larvae, but did not affect the corresponding excision strain. While extra Hsp70 provides additional protection against the immediate damage from heat stress, abnormally high concentrations can decrease growth, development and survival to adulthood.