A new concept of photothermal (PT) lifetime imaging (PTI) is suggested. This technique is based on heating absorbing intracelular chromophores with a short laser pulse, together with time-resolved monitoring of their cooling. The process is monitored with phase-contrast imaging of a second coaxial probe pulse. PTI enables the estimation of the average size of absorbing targets by measuring their cooling times. Resolving overlapping absorption targets of varying sizes can be accomplished by adjusting the time delay between excitation and probe pulses. The pharmaceutical application of PTI is based on the assumption that drug action, through different biochemical processes, could change some properties of endogenous chromophores (absorption, sizes, etc.) as natural markers. This can lead to corresponding changes in PT signal parameters (amplitude, cooling time etc.). The ability of PTI to obtain information about a drug's impact is discussed, with emphasis on PT monitoring of alterations in the cellular absorbing ultrastructure. Preliminary experimental data are presented as PT images of blood cells in the presence of a drug obtained with a pulsed Nd:YAG laser (8 ns, 532 nm, 1-100 (mu) J). Other potential applications of PTI operating in lifetime mode include guidance of laser cellular microsurgery, visualization of local temperature effects and study of nano-scale structures.