A new series of 2-mercapto-3-substituted-5,6-dimethylthieno[2,3-d]pyrimidin-4(3H)-ones were synthesised by reacting 3-amino-2-mercapto-5,6-dimethylthieno[2,3-d]pyrimidin-4(3H)-one with different aldehydes and ketones. The starting material 3-amino-2-mercapto-5,6-dimethylthieno[2,3-d]pyrimidin-4(3H)-one was synthesised from 2-amino-3-carbethoxy-4,5-dimethyl thiophene by a novel innovative route. The title compounds were investigated for analgesic, anti-inflammatory and ulcerogenic index activities. While the test compounds exhibited significant activity, among these compound AS1 showed more potent analgesic and anti-inflammatory activities and the compound AS3 showed equipotent analgesic and anti-inflammatory activities when compared to the reference standard diclofenac sodium.