OBJECTIVE Photothermal sensitisation has been recently proposed as a novel approach for the treatment of solid tumours through the development of a therapeutic modality named photothermal therapy (PTT). The technique involves the use of high power pulsed laser irradiation and photosensitising agents with especially short lifetime (in the subnanosecond range) in the electronically excited states. This study aims to investigate the molecular features of the photosensitiser which optimise the photothermal activity. METHODS Two octabutoxy-naphthalocyanines centrally coordinated with Pd(II) or Pt(II) ions were prepared by chemical synthesis and tested for their affinity and photothermal sensitisation activity toward a selected tumour cell line, namely B78H1 amelanotic melanoma. Irradiations were performed by using a Ti:sapphire laser operated in a pulsed regime (10 Hz, 30 nanosecond pulses, 120 mJ) at 809 nm (Pt) or 826 nm (Pd). The subcellular distribution pattern of the photosensitiser was also assessed by optical microscopy, while the nature of the photoinduced cell damage was determined by scanning electron microscopy. The results thus obtained provided a basis for subsequent in vivo studies, aimed at defining the phototherapeutic efficiency of the two metallo-naphthalocyanines: the photosensitisers were i.v. injected into C57BL/6 mice bearing a subcutaneously transplanted amelanotic melanoma and at 24 hours post-injection the tumour area was irradiated by the Ti:sapphire laser using the same protocol as above detailed. RESULTS Both naphthalocyanines exhibited a high affinity for the amelanotic melanoma cells. The subcellular distribution pattern was modulated by the incubation time: after 48 hours incubation with 7.7 microM Pd- and Pt derivatives, the naphthalocyanine appeared to localise in specific sites with a gradual formation of aggregated clusters. Subsequent irradiation of the naphthalocyanine-loaded cells caused an extensive cell death; the photoinduced damage, as observed at the scanning electron microscope, mainly consisted in the formation of large endocellular holes consequent to the loss of cytoplasmic material. This scenario is typical of photothermal sensitisation processes. Lastly, both metallo-naphthalocyanines, and in particular the Pd(II) derivative, promoted an important response by the amelanotic melanoma, when the neoplastic tissue was irradiated by the pulsed Ti:sapphire laser. In certain cases, the photothermal treatment appeared to be curative. In all cases, the in vivo photodamage was confined within the tumour area with no detectable involvement of the perilesional tissues. CONCLUSIONS PTT appears to act very efficiently at least on subcutaneous tumours. The technique can be used either in combination with photodynamic therapy (PDT) or as an alternative to PDT in those cases where the latter modality displays a limited efficacy, such as in the treatment of pigmented or poorly vascularised tumours.