The growth and development of a plant depend on the intensity, quality and duration of the solar radiation. The present work had for objective to evaluate solar radiation variations, such as the availability inside a plastic greenhouse with tomato cropped in summer-autumn seasons in Pelotas, Rio Grande do Sul State, Brazil. The experiment was carried out from January to June 2003, at the Federal University of Pelotas (31°52'S; 52°21'W; 13m alt.), inside a plastic greenhouse, with an area of 180m², placed in the East-west direction. The Flora-dade cultivar was sown on January 24, transplanted on February 28, and harvested on June 12. The global solar radiation outside and inside, transmissibility, photosynthetically active radiation (PAR) and albedo were measured with electronic sensors connected to a datalloger. Throughout the crop cycle, the total of global solar radiation outside the greenhouse was 1161.21MJ m-2, while inside it was 881.85MJ m-2. The average daily values of inside global solar radiation and PAR were 8.5MJ m-2 dia-1 and 3.4MJ m-2 dia-1, respectively. The mean transmittance of the greenhouse for the global solar radiation was 76%. Daily mean value of albedo was 0.23, with 0.17 in the beginning of the culture development, 0.26 at maximum development and 0.23 at the end of the cycle.