Gamma-rays have been applied in an NDT campaign aimed at evaluating the condition and the exact location of the internal post-tensioning cables of an existing box-girder concrete bridge. Previous visual inspections to the bridge deck revealed systematic cracking patterns and localized corrosion signs in the post-tensioning ducts. In this paper, the test logistics is described as well as the main results that were obtained. INTRODUCTION The N. S. da Guia Bridge (Fig. 1) is a prestressed concrete bi-cellular box-girder bridge with a total length of 250m, divided by 5 spans of 38-3x58-38m. The cross-section height varies parabolically from 1.45m at the spans to 2.9m at the intermediate supports with a resulting slenderness ranging between L/40 and L/20. The deck is 11.84m width supporting one carriageway with two traffic lanes and being supported by laminated neoprene bearing pads which rest on lightly reinforced concrete piers. This was one of the first bridges designed and built in Portugal according to the segmental cantilever construction method. Although the original design dates back to 1973, the construction only began in 1978 and the bridge was opened to traffic in 1980. This bridge is located in the national road EN 201, which was once part of the main route connecting the Spanish region of Galicia with the North-western part of Portugal. In 1998 a new motorway connecting these two regions was opened and nowadays the bridge is crossed mainly by local traffic. During a routine inspection several cracks were detected in the webs which were not reported in previous inspections. A comprehensive examination program was developed aimed at evaluating: the causes of the observed cracking patterns; the current safety level of the bridge, and; the urgency of eventual strengthening actions. As part of this program, a detailed visual inspection campaign was carried out enabling a complete mapping of the cracking patterns and other anomalies of the bridge deck. Both internal and external inspections were performed. Consistent cracking patterns were observed in the webs and in the bottom slab, exhibiting both longitudinal and transversal symmetry, as depicted in Fig. 2. The bridge was monitored both during a load test and under normal serviceability conditions during a period of one week. Although the cracks exhibited opening a closing movements, no significant deviation could be observed from the calculations performed using a linear elastic model considering uncracked concrete behaviour [8]. In a limited number of predefined locations, a covermeter was used to check if the reinforcement layout coincided with the prescribed in the original design. The agreement was found to be good. Only the most superficial reinforcements, and not the post-tensioning ducts, could be detected with this method. The concrete strength was evaluated by combining the results of NDT methods (sclerometer test and ultra-sounds) with a limited number of tests on drilled cores.