Drinking lots of water but always thirsty. not urinating excessivly. what could it be?

Many possibilities.: Could be a variety of possible causes. On example may be a diabetic with poor kidney funcion.

Diabetes is concern: This is a very vague complaint- but many people present with this problem when there blood sugars are running too high. The excess urination may not always be present. Simple dehydration is also possible- this will stimulate your thirst but you will not urinate a lot b/c your body needs the fluids. There are other possible conditions but it would be best to have other symptoms clarified by dr.

Polydipsia: You have polydipsia, which is defined as excessive thirst combined with drinking more than normal. If you are chronically dehydrated, you may be more thirsty than normal. You may need to see your doctor to to rule out diabetes mellitis (high sugar levels) or diabetes insipidis (abnormality in the kidney's excretion), but those are normally also associated with increased urination.