This work aimed to evaluate and correlate the thermoregulatory characteristics of crossbredcows (Holstein x Gir) submitted to three different periods of exposure to solar radiation with somemeteorological variables and their effects on milk production. Were twelve lactating cows dividedinto three groups consisting of four animals each, all of the third lactation, up to five years of ageand approximately 50 days of lactation, submitted to different periods of exposure to solar radiation.Milk production per cow in kilograms was recorded in two daily milkings. The respiratory rate (RR),heart rate (RR), rectal temperature (RT) and the decline in milk production (DPL) obtained bythe difference between the weights of the morning and afternoon milkings per cow during theexperiment were collected four times a week in May and June 2011. The results revealed that thetemperature and humidity index (THI) for cows exposed to two hours of solar radiation was higherthan values considered critical for lactating cattle. The group presented three upper critical valuesfor all thermoregulatory characteristics of animals when compared with groups one and two.The thermoregulatory characteristics of animals of groups two and three groups were positivelycorrelated with the decline in milk production and differently with meteorological variables.Este trabalho teve o objetivo de avaliar e correlacionar as características termorreguladoras devacas mestiças (Holandês x Gir) submetidas a três períodos diferentes de exposição à radiação solarcom algumas variáveis meteorológicas e seus efeitos na produção de leite. Foram utilizadas dozevacas lactantes divididas em três grupos compostos por quatro animais cada, todos de terceiralactação, acima de cinco anos de idade e aproximadamente 50 dias de lactação, submetidosa diferentes períodos de exposição à radiação solar. A produção de leite em quilogramas porvaca foi registrada em duas ordenhas diárias. A frequência respiratória (FR), a frequência cardíaca(FR), a temperatura retal (TR) e o declínio da produção de leite (DPL) obtido pela diferença entreas pesagens das ordenhas da manhã e tarde por vaca durante o experimento foram coletadosquatro vezes por semana, em maio e junho de 2011. Os resultados revelaram que o índicetemperatura e umidade (ITU) para as vacas expostas a duas horas de radiação solar foi superior avalores considerados críticos para bovinos em lactação. O grupo três apresentou valores críticossuperiores para todas as características termorreguladoras dos animais quando comparada comos grupos um e dois. As características termorreguladoras dos animais dos grupos dois e trêscorrelacionaram-se positivamente com o declínio da produção de leite e de maneira diferentecom as variáveis meteorológicas. Effect of exposure to solar radiation on physiological parameters anddecline estimation of milk production by crossbred cows(Holstein x Gir) in the southern Piauí State, BrazilThis work aimed to evaluate and correlate the thermoregulatory characteristics of crossbredcows (Holstein x Gir) submitted to three different periods of exposure to solar radiation with somemeteorological variables and their effects on milk production. Were twelve lactating cows dividedinto three groups consisting of four animals each, all of the third lactation, up to five years of ageand approximately 50 days of lactation, submitted to different periods of exposure to solar radiation.Milk production per cow in kilograms was recorded in two daily milkings. The respiratory rate (RR),heart rate (RR), rectal temperature (RT) and the decline in milk production (DPL) obtained bythe difference between the weights of the morning and afternoon milkings per cow during theexperiment were collected four times a week in May and June 2011. The results revealed that thetemperature and humidity index (THI) for cows exposed to two hours of solar radiation was higherthan values considered critical for lactating cattle. The group presented three upper critical valuesfor all thermoregulatory characteristics of animals when compared with groups one and two.The thermoregulatory characteristics of animals of groups two and three groups were positivelycorrelated with the decline in milk production and differently with meteorological variables.